MB-100: avoid hanging on checking verify_cluster_stats

Change-Id: I645f79c4e51fe27409efb4c0a1c547287c32592c
Reviewed-on: http://review.couchbase.org/24018
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>