MB-12476: Set original MBB correctly

The RR*-tree algorithm uses a so-called "original MBB". It is the
bounding box of the node when it was first created.

Change-Id: I24c5f57a0dacb352c3f2777b0c1a23e0ac15ac90
Reviewed-on: http://review.couchbase.org/42754
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>