MB-11638: update highseqno stat periodically

As part of xdcr_opaque_checker we now do periodic vbucket-seqno stat
requests for source bucket and inform vbucket replicators if vbucket's
highseqno becomes bigger. Check is done every 60 seconds.

Change-Id: Ib2420f680063e859c7d3a2dc4881d052f3866965
Reviewed-on: http://review.couchbase.org/40508
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>