OAK-593: Segment-based MK

Add a special "diff" map record type for more efficient handling of the
common case where just a single entry that already exists in the base map
gets modified.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1550530 13f79535-47bb-0310-9956-ffa450edef68