Split up artifact cache and CAS cache
Description
This is a backport of !922 (merged) (and !882 (merged)) for bst-1.
This was mainly a cleanup to allow further development, but it had the nice side effect that CASServer no longer had to create a Context to get the artifact cache, and could use CASCache directly.
Using CASCache directly allows for instant startup of bst-artifact-server, which otherwise would have to wait until the context determines the size of the cache directory (which is useless as the CASServer has its own CacheCleaner).