OAK-1032: SegmentMK: One SegmentWriter per SegmentNodeStore

Simplify segment creation as an initial step. WIP... (has some negative performance impact)

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1526277 13f79535-47bb-0310-9956-ffa450edef68