Expire artifacts in the local cache

This merge request includes and replaces !347 (closed), find all relevant information in !347 (closed) (issue #135 (closed))

Basically this merge includes some additional fixes on top of @tlater's branch, mostly so as to retain clean and streamlined logging code paths.

merged

mentioned in merge request !347 (closed)

mentioned in issue #479 (closed)

mentioned in commit buildstream-docker-images@ae7ff8e1

mentioned in merge request buildstream-docker-images!50 (merged)

mentioned in commit buildstream-docker-images@847ab3c6

changed the description