MB-8746: Add support for incremental updates

Now items can be updated/delete from the spatial index.

Cleanup currently isn't supported.

Change-Id: I514947960e9e12bd5aed7d24277884392058535d
Reviewed-on: http://review.couchbase.org/28077
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>