Log reference updates made by fetches

Fetches use a workaround for capturing the reference changes being made and voting for them. The reference changes that apply to the repository are done with update-ref. This flow needs updating to go through the write-ahead log.

These updates need to go through the single-writer flow for verification, logging and application. The references should be fetched to some internal namespace so the reference locks created when invoking reference transaction hook do not block the repository worker from making progress.

The issue is written with the implementation from #3780 (closed) in mind.