Makes artifactdir and builddir obsolete.

Fixes #870

Will check and move old artifact directory if it exists, and create
symlink linking old directory to new.

Part of #870

This sits in Context allowing artifact cache to check the cas quota
while not being used for CASServer.

A lot of code that checks cache quota has been touched.

Part of #870

ArtifactCache now derives from BaseCache, and so will the SourceCache.
Methods relevant to both have been moved here, such as generating specs
from the config.

WIP

Part of #440

This introduced the source cache which is used in sources to avoid full
staging and will allow fetching of sources from a configured remote CAS.

Part of #440

Very much not an ideal solution but gets source cache working.

Part of #870