MB-11866 Start updater only if an updater isn't running

When an async stats response is processed by the view group,
it needs to check if an updater is already running before starting
an updater instance. Otherwise multiple updater instances could be
started and lead to index data file corruption.

Change-Id: Ib4e3f0abb44e6babfe6b53b4f2471d865dcfd5d7
Reviewed-on: http://review.couchbase.org/40260
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: buildbot <build@couchbase.com>