Speed up `bst shell` command

bst shell is currently much too slow. It would be nice to aggressively attempt to speed things up. I know this is never going to be instantaneous, like jhbuild shell, but I hope we we can do a lot better here.

For instance:

$ bst shell --build core/epiphany.bst
[--:--:--][][] START   Loading pipeline
[00:00:00][][] SUCCESS Loading pipeline
[--:--:--][][] START   Resolving pipeline
[00:00:00][][] SUCCESS Resolving pipeline
[--:--:--][][] START   Resolving cached state
[00:01:35][][] SUCCESS Resolving cached state
[--:--:--][][] START   Staging dependencies
[--:--:--][????????][ main:core/epiphany.bst             ] WARNING Non-whitelisted overlaps detected

    Staged files overwrite existing files in staging area:
    /usr/bin/gtkdoc-check: core-deps/gtk-doc.bst is not permitted to overlap other elements, order core-deps/gtk-doc.bst above base/base-configure.bst 
    /usr/bin/gtkdoc-depscan: core-deps/gtk-doc.bst is not permitted to overlap other elements, order core-deps/gtk-doc.bst above base/base-configure.bst 
    /usr/bin/gtkdoc-fixxref: core-deps/gtk-doc.bst is not permitted to overlap other elements, order core-deps/gtk-doc.bst above base/base-configure.bst 
    /usr/bin/gtkdoc-mkdb: core-deps/gtk-doc.bst is not permitted to overlap other elements, order core-deps/gtk-doc.bst above base/base-configure.bst 
    /usr/bin/gtkdoc-mkhtml: core-deps/gtk-doc.bst is not permitted to overlap other elements, order core-deps/gtk-doc.bst above base/base-configure.bst 
    /usr/bin/gtkdoc-mkman: core-deps/gtk-doc.bst is not permitted to overlap other elements, order core-deps/gtk-doc.bst above base/base-configure.bst 
    /usr/bin/gtkdoc-mkpdf: core-deps/gtk-doc.bst is not permitted to overlap other elements, order core-deps/gtk-doc.bst above base/base-configure.bst 
    /usr/bin/gtkdoc-rebase: core-deps/gtk-doc.bst is not permitted to overlap other elements, order core-deps/gtk-doc.bst above base/base-configure.bst 
    /usr/bin/gtkdoc-scan: core-deps/gtk-doc.bst is not permitted to overlap other elements, order core-deps/gtk-doc.bst above base/base-configure.bst 
    /usr/bin/gtkdoc-scangobj: core-deps/gtk-doc.bst is not permitted to overlap other elements, order core-deps/gtk-doc.bst above base/base-configure.bst 
    /usr/bin/gtkdocize: core-deps/gtk-doc.bst is not permitted to overlap other elements, order core-deps/gtk-doc.bst above base/base-configure.bst 
    /usr/lib/cmake/GtkDoc/GtkDocConfig.cmake: core-deps/gtk-doc.bst is not permitted to overlap other elements, order core-deps/gtk-doc.bst above base/base-configure.bst 
    /usr/lib/cmake/GtkDoc/GtkDocConfigVersion.cmake: core-deps/gtk-doc.bst is not permitted to overlap other elements, order core-deps/gtk-doc.bst above base/base-configure.bst 
    /usr/lib/cmake/GtkDoc/GtkDocScanGObjWrapper.cmake: core-deps/gtk-doc.bst is not permitted to overlap other elements, order core-deps/gtk-doc.bst above base/base-configure.bst 
    /usr/share/aclocal/gtk-doc.m4: core-deps/gtk-doc.bst is not permitted to overlap other elements, order core-deps/gtk-doc.bst above base/base-configure.bst 
    /usr/share/gtk-doc/data/devhelp2.xsd: core-deps/gtk-doc.bst is not permitted to overlap other elements, order core-deps/gtk-doc.bst above base/base-configure.bst 
    /usr/share/gtk-doc/data/devhelp2.xsl: core-deps/gtk-doc.bst is not permitted to overlap other elements, order core-deps/gtk-doc.bst above base/base-configure.bst 
    /usr/share/gtk-doc/data/gtk-doc.flat.make: core-deps/gtk-doc.bst is not permitted to overlap other elements, order core-deps/gtk-doc.bst above base/base-configure.bst 
    /usr/share/gtk-doc/data/gtk-doc.make: core-deps/gtk-doc.bst is not permitted to overlap other elements, order core-deps/gtk-doc.bst above base/base-configure.bst 
    Message contains 6766 additional lines

[00:00:12][][] SUCCESS Staging dependencies
[--:--:--][][] START   Integrating sandbox
[--:--:--][593e35e8][ main:base.bst                      ] STATUS  Running integration command

    ldconfig

[--:--:--][593e35e8][ main:base.bst                      ] STATUS  Running integration command

    PKGSYSTEM_ENABLE_FSYNC=0 update-mime-database /usr/share/mime

[--:--:--][593e35e8][ main:base.bst                      ] STATUS  Running integration command

    update-desktop-database -v

Search path is now: [/usr/local/share/applications, /usr/share/applications]
Could not create cache file in "/usr/local/share/applications": Error opening directory ?/usr/local/share/applications?: No such file or directory
File "/usr/share/applications/gtk3-widget-factory.desktop" lacks MimeType key
File "/usr/share/applications/gcr-prompter.desktop" lacks MimeType key
File "/usr/share/applications/python3.6.desktop" lacks MimeType key
File "/usr/share/applications/geoclue-where-am-i.desktop" lacks MimeType key
File "/usr/share/applications/geoclue-demo-agent.desktop" lacks MimeType key
File "/usr/share/applications/gtk3-demo.desktop" lacks MimeType key
File "/usr/share/applications/texdoctk.desktop" lacks MimeType key
File "/usr/share/applications/gtk3-icon-browser.desktop" lacks MimeType key
File "/usr/share/applications/python2.7.desktop" lacks MimeType key
[--:--:--][ec9dc658][ main:core-deps/glib.bst            ] STATUS  Running integration command

    glib-compile-schemas /usr/share/glib-2.0/schemas

Warning: Schema “org.gnome.crypto.pgp” has path “/desktop/gnome/crypto/pgp/”.  Paths starting with “/apps/”, “/desktop/” or “/system/” are deprecated.
Warning: Schema “org.gnome.system.locale” has path “/system/locale/”.  Paths starting with “/apps/”, “/desktop/” or “/system/” are deprecated.
Warning: Schema “org.gnome.system.proxy” has path “/system/proxy/”.  Paths starting with “/apps/”, “/desktop/” or “/system/” are deprecated.
Warning: Schema “org.gnome.system.proxy.http” has path “/system/proxy/http/”.  Paths starting with “/apps/”, “/desktop/” or “/system/” are deprecated.
Warning: Schema “org.gnome.system.proxy.https” has path “/system/proxy/https/”.  Paths starting with “/apps/”, “/desktop/” or “/system/” are deprecated.
Warning: Schema “org.gnome.system.proxy.ftp” has path “/system/proxy/ftp/”.  Paths starting with “/apps/”, “/desktop/” or “/system/” are deprecated.
Warning: Schema “org.gnome.system.proxy.socks” has path “/system/proxy/socks/”.  Paths starting with “/apps/”, “/desktop/” or “/system/” are deprecated.
[--:--:--][81d1654d][ main:core-deps/gdk-pixbuf.bst      ] STATUS  Running integration command

    gdk-pixbuf-query-loaders --update-cache

[--:--:--][0566cf06][ main:core-deps/gtk+-3.bst          ] STATUS  Running integration command

    for dir in /usr/share/icons/*; do
      if test -f $dir/index.theme; then
        gtk-update-icon-cache --quiet $dir
      fi
    done

[--:--:--][0566cf06][ main:core-deps/gtk+-3.bst          ] STATUS  Running integration command

    gtk-query-immodules-3.0 --update-cache

[00:00:14][][] SUCCESS Integrating sandbox
[--:--:--][][] START   Staging sources
[00:00:00][][] SUCCESS Staging sources
[--:--:--][????????][ main:core/epiphany.bst             ] STATUS  Running command

    bash --noprofile --norc -i

Here we spent:

• 1m35s resolving cached state
• 12s staging dependencies
• 14s integrating sandbox

Looks like "resolving cached state" is clearly the first thing to focus on.