CBD-125: Make real use of the "Original MBB"

According to the RR*-tree paper, you should store the "original MBB",
which is the MBB at the creation time of the node, with the node.

Change-Id: Idbff371680affe7f049861e862dd4d3da8c030ae
Reviewed-on: http://review.couchbase.org/21028
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>