housekeeping: Prepare for reftables

The MR adds the reference backend type to the repository stats structure, with this we don't check for loose refs or packed refs when the repo is using the reftable backend for writing commit graphs. We fix tests around this.

We don't add and code towards cleanup of reftable locks because this will be handled by the transaction manager and since each log entry is independent and isolated, we won't have any stale reftable locks left in the repo.

The other section of integration here is the integration with transaction manager and how to run housekeeping with the transaction manager. It's still not clear what the best way here would be, and it might be that we disable auto compaction of reftables and handle it via the housekeeping process which runs as part of the transaction manager.

Closes #5767 (closed)