MB-11299: Use the snapshot start seqno when creating a failover entry

Change-Id: I7a2c0adf51a761a491d0f12720f2abef8127e5c3
Reviewed-on: http://review.couchbase.org/40341
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>