OAK-631: SegmentMK: Implement garbage collection

Make the SegmentIdFactory keep weak references to all segment identifiers,
so we can tell which ones are still being accessed by existing sessions.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1559709 13f79535-47bb-0310-9956-ffa450edef68