This is what the whole resource.py thing was created for, the
cleanup job must have exclusive access to the cache, while the pull
and build jobs which result in adding new artifacts, must only require
shared access.

This is a part of #623

This means there is no cap for shared resource requests.

Together with the previous commit, this causes the cleanup
job and the pull/build jobs to all require unlimited shared
access to the CACHE resource; while only the cleanup job
requires exclusive access to the resource.

This is a part of #623

This runs after every pull, and does not need the cache exclusively,
only the cleanup job requires the cache exclusively.

Without this, every time a cache_size job is queued, all pull and
build jobs need to complete before cache_size job can run exclusively,
which is not good.

This is a part of #623