EPEngine::destroy: explain why stats are snapshotted
EPEngine::destroy: explain why stats are snapshotted

Change-Id: Iddf82827efc7810c2fb6abd459d31e777fcc7392
Reviewed-on: http://review.couchbase.org/76466
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>

daverigby/kv_engine,couchbase/ep-engine,membase/ep-engine,daverigby/kv_engine,membase/ep-engine,couchbase/ep-engine,daverigby/kv_engine,couchbase/ep-engine,daverigby/kv_engine,membase/ep-engine,membase/ep-engine,couchbase/ep-engine