OAK-631: SegmentMK: Implement garbage collection

GC is best implemented below the SegmentStore interface, not above it,
so we can drop the deleteSegment() method

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1574627 13f79535-47bb-0310-9956-ffa450edef68