MB-8746: Preparation for spatial incremental update

This commit prepares the support for incremental updates of
spatial indexes. This commit adds the needed functionality
within the couchdb project, but the code path isn't used yet.
It will be used, once the set view based spatial views are
used instead of the old ones.

Change-Id: Ie26d1420fecfc32671efb494927eed733ac5f74a
Reviewed-on: http://review.couchbase.org/28063
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>