OAK-593: Segment-based MK

Fix another deadlock in FileStore.close vs. the flush thread:
the flushThread.join() call must not be within a synchronized block.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1548137 13f79535-47bb-0310-9956-ffa450edef68