MB-7406: Add updater barrier for spatial indexer

Currently there's one spatial indexer for every vBucket,
hence a lot of updaters are running at the same time.
This commit adds a barrier, so that you can set the
maximum number of updaters that should run at the same
time. The default value is 4. The MapReduce indexer
already uses such a barrier.

Those two commits should me merged at the same time:
http://review.couchbase.org/24830
http://review.couchbase.org/24831

Change-Id: Ie1e2a5b2f50cfa1137ec24ed414e9d8a4adac4d2
Reviewed-on: http://review.couchbase.org/24830
Tested-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Fulu Li <fulu@couchbase.com>
Reviewed-by: Fulu Li <fulu@couchbase.com>