MB-11843 Fix error case handling for async stats response reader

For stats_cache, if waiting=true, no other async stats requests
are scheduled. In case such as dcp connection close, async stats
response will be error. In such case, we should reset waiting flag
to false so that next query request can trigger a stats request.

Change-Id: I4dd9df199d4a8cccdfd84a52c0eb64ec5bf78c61
Reviewed-on: http://review.couchbase.org/40257
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: buildbot <build@couchbase.com>