Support reftables in transaction management & housekeeping

The initial issue to add reftable support in housekeeping doesn't consider the movement of housekeeping into the transaction manager.

As we move housekeeping to the transaction manager, it is important that reftable support is considered there too. Here we want housekeeping to take over reftable autocompaction. This in itself means that:

1. We need to disable the autocompaction feature of reftables (needs to be modified in Git)
2. Each transaction can autocompact new tables it creates, but shouldn't modify existing tables in the repo (needs to be modified in Git).
3. We compact all reftable tables as part of the housekeeping task running within the transaction manager.