OAK-593: Segment-based MK

Make MemoryStore implement SegmentStore directly to make it easier to simplify caching in AbstractStore.
Drop SegmentStore.isInstance().

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1575339 13f79535-47bb-0310-9956-ffa450edef68