OAK-1032: SegmentMK: One SegmentWriter per SegmentNodeStore

Make the currently built segment accessible through readSegment() so we don't need to flush so often.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1526512 13f79535-47bb-0310-9956-ffa450edef68