MB-10788: Implement observe_seqno API

Change-Id: I9bf4adc1492cd55df850a52fe7ca4b35dc1eb4c1
Reviewed-on: http://review.couchbase.org/44034
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Sriram Ganesan <sriram@couchbase.com>