WIP: Add `--limit-queries` option for Musicbrainz

The idea here is: I want to make a playlist using Musicbrainz metadata, but I don't want to query Musicbrainz 100000 times on first run.

Since there is caching, we could limit queries to 100 and slowly build up a cache making better and better playlists each time. It might work!

This implementation may not be the best one. It breaks some assumptions about Cache.wrap() which can now return None; although perhaps callers should just handle that case? As the underlying API call might anyway fail.