OAK-593: Segment-based MK
OAK-1283: TCK tests slow on SegmentMK+Mongo

Drop the currently unused MongoStore to make refactorings easier.
We can resurrect the MongoDB backend for the SegmentMK later on
if a need arises.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1572794 13f79535-47bb-0310-9956-ffa450edef68