Reduce max number of numbers in the MBB to 16-bit

In order to make the on-disk file format of the spatial views more
along the lines of the mapreduce views, reduce the number of numbers
in the MBB from 32-bit to 16-bit.

Change-Id: Ia00f88df33d2ab1b1bd2f6f39348072f7d617f4e
Reviewed-on: http://review.couchbase.org/39633
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>