Move MaxInfoHashes and MaxInfoHashPeers to the Config.

Apologies if this broke anyone's code. Please use the new API based on Config.

This change should be functionality a no-op.
Fix various bugs in the peer store.

- the vitess LRU in-place value update isn't working for some reason. It's
  weird because upstream has working tests for this. Anyway I got tired and
  switched to another library.
- found an off-by-one error when checking for the number of contacts for an
  infohash - the code was allowing an extra entry.

Things are properly tested now, so it should be easier now to do surgery on
this code when the time comes.