Merge branch 'raoul/cas-refactor' into 'master'

buildstream/_artifactcache.py

buildstream/_cas/__init__.py

buildstream/_cas/cascache.py

buildstream/_cas/casremote.py

buildstream/_cas/casserver.py

buildstream/_context.py

buildstream/_exceptions.py

buildstream/sandbox/_sandboxremote.py

doc/source/using_configuring_artifact_server.rst

tests/artifactcache/config.py

tests/artifactcache/expiry.py

tests/sandboxes/storage-tests.py

tests/storage/virtual_directory_import.py

tests/testutils/artifactshare.py

tests/utils/misc.py