Fixes #632.

_artifactcache/casserver.py: Implement BatchReadBlobs

Closes #632

See merge request !785

source/install_source.rst: pip plugin depends on host pip

See merge request !791

The first badge will work fine right away while the second badge will
show "not found" until a release is made after merging this branch:
!718.

README.rst: Add status badges for PyPI release and Python versions

See merge request !719

richardmaw-codethink authored Sep 05, 2018 and richardmaw-codethink committed Sep 13, 2018

This can get out of sync with other two cache states,
and we can do without it.

Simplify element state by removing `__cached`

See merge request !784

We can't include a socket in a CAS tree, but it's mostly meaningless to do so
since there can't possibly be a process serving it.