OAK-1526: Lock contention in SegmentIdFactory

Rename SegmentIdFactory to SegmentTracker and move the segment cache to it.
Make SegmentId.getSegment() unsynhronized for the common case

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1577772 13f79535-47bb-0310-9956-ffa450edef68