MB-15990: KVShard::setLowPriorityVbSnapshotFlag not working as expected.
MB-15990: KVShard::setLowPriorityVbSnapshotFlag not working as expected.

The flag is not set because on the compare_exchange if flag equals
!newvalue, then flag is set to original value, as opposed to the
newvalue.

Change-Id: I029a4d90d601ff53a41926e2806cf9440783ac5c
Reviewed-on: http://review.couchbase.org/54218
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>
Reviewed-by: Chiyoung Seo <fcffc23db27b19ee4ebba46695ee6b50e09216a2@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,membase/ep-engine,hisundar/ep-engine,daverigby/kv_engine,hisundar/ep-engine,daverigby/ep-engine,daverigby/ep-engine,couchbase/ep-engine,owendCB/ep-engine,daverigby/kv_engine,owendCB/ep-engine,couchbase/ep-engine,hisundar/ep-engine,teligent-ru/ep-engine,couchbase/ep-engine,teligent-ru/ep-engine,membase/ep-engine,hisundar/ep-engine,membase/ep-engine,teligent-ru/ep-engine,daverigby/ep-engine,daverigby/ep-engine,couchbase/ep-engine,owendCB/ep-engine,teligent-ru/ep-engine,membase/ep-engine,owendCB/ep-engine