Refine forced compaction

Forced compaction currently acquires an exclusive write lock on the repository blocking all concurrent commits during the complete time it needs to finish compaction. I think we should refine this:

• Add a time out so we could limit the time during which the repository does not accept writes while still giving compaction another chance to finish.

• Boost the compaction threads priority. This could actually already be done during the regular compaction cycles to increase the changes to finish in time.