Support a periodic AC expiration policy on missing CAS entries

[shirchen]

Idea was mentioned in https://buildteamworld.slack.com/archives/C9C4H1SN7/p1622229950018700 where when using buildfarm for cache only, there is no way to intelligently expire CAS entries when workers will unavoidably fill up.

One idea is a simple background job that scans the Redis space for all AC entries and if associated CAS entry is missing, expire the AC entry.

[luxe]

That's a good idea. Sounds easy enough. I need to check if there is an API for asking a worker whether they have a blob or whether the blob has to actually be fetched in order to find out.