OAK-1927: TarMK compaction delays journal updates

Better not to track backlog since the compaction
already automatically rebases recent changes and
flush is invoked once every five seconds.

Also expose more details in the thread name

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1607196 13f79535-47bb-0310-9956-ffa450edef68