MB-10788: Implement observe-seqno API

Change-Id: I883603228a8a208632eeb9d1986f5776de6618ba
Reviewed-on: http://review.couchbase.org/44120
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: buildbot <build@couchbase.com>
Merge remote-tracking branch 'couchbase/master'

* couchbase/master:
  MB-10788: Implement observe-seqno API

Change-Id: I5d8601cafc828fd9ded26efe6376c6d8cbde9d99