Transmit the enclosing bounding box

For the initial index build it is needed to know the enclosing bounding
box in order to build up the tree bottom up.

Change-Id: I3b625f46ae840dc36f2b6499fb1daf67c4efadb8
Reviewed-on: http://review.couchbase.org/40136
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>