Draft: Move artifact and source cache queries to scheduled jobs

This moves artifact and source cache queries to scheduled job threads, in preparation for 'remote execution without the bytes' where remote cache queries will be necessary to determine the cache status of an artifact. It is based on top of the threaded scheduler (!1982 (merged)) as that reduces the overhead of the extra jobs. The complete test suite passes, however, there are still a few points to be discussed and finalized. I'll open discussion threads as appropriate.

This MR depends on !1982 (merged) and !2084 (merged).