OAK-593: Segment-based MK

It's best to assume that there were no other concurrent changes when merging a branch. If there weren't, we avoid an extra getHead() roundtrip. If there were, we try again a bit later after rebasing.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1452855 13f79535-47bb-0310-9956-ffa450edef68