MB-11809: Don't use last_by_seqno when checking for sending marker ack

The last_by_seqno variable is set when an item is received and
buffered. As a result it is possible that the last_by_seqno might
be different than the mutation seqno we are surrenty processing. This
can lead to no sending an ack when we should.

Change-Id: I0fe43e60b51d2050ab5c8bea016622a7ac0affc7
Reviewed-on: http://review.couchbase.org/39894
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>