CBQE-928: cancel thread in _verify_stats_all_buckets if failure

apparently because of this test are hanging in http://qa.hq.northscale.net/view/2.0.1/job/centos-64-2.0-upgrade/51/consoleFull
in online_upgrade_incremental test

Change-Id: Idd14bdfd601dabb01f59f12a6008dca6b007dcd8
Reviewed-on: http://review.couchbase.org/23821
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>