MB-11383: When thresholds are met, set warmup to complete

With parallelization, it is possible that other warmup
threads would not respect the completion state that is
marked by one thread. This fix, would ensure that other
threads will also respect the completion state set by
one thread as best possible.

Change-Id: Ie3223e5a95ac5f3c3281e18184c0cebf63fe993f
Reviewed-on: http://review.couchbase.org/40062
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>