Reftable: allow compaction even when there are locked tables

The reftable backend, auto compacts tables as and when it is necessary. However, when we're using in the context of Gitaly, it will be used alongside the WAL.

The WAL records file changes and not reference update changes in its logs. So this means, we cannot autocompact reftable files. This is because the existing files of the reftable are recorded and only new files will be added to the log. But this doesn't stop us from compacting the new tables which are to be added.

Currently, when few tables are locked, none of the tables are compacted. It would be nice if the reftable logic was a bit more dynamic, wherein it would skip locked tables and compact the remaining tables. This ensures we don't accumulate a bunch of new tables and also ensures that maintenance is more spread out.