OAK-1031: SegmentMK: Fewer segment lookups

Include a Segment reference in the Record base class to avoid extra lookups.
The reference is lazily adjusted to point to the correct segment to avoid
potentially costly pre-loading of all referenced segments that might not
end up being read at all.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1526651 13f79535-47bb-0310-9956-ffa450edef68