OAK-631: SegmentMK: Implement garbage collection

Add checksums to tar entries for better sanity checks during recovery
Write new tar files during cleanup if doing so would save at least 25% space

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1583718 13f79535-47bb-0310-9956-ffa450edef68