OAK-1031: SegmentMK: Fewer segment lookups

Return MapEntry instead of RecordId from MapRecord.getEntry(String) so that we can keep the correct Segment reference along with the return value

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1528529 13f79535-47bb-0310-9956-ffa450edef68