MB-5177: added hibernated connections stats

Change-Id: Id5c4ea9105a6f137dbc4fe14676216230b903682
Reviewed-on: http://review.couchbase.org/30138
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>