Make sure seqno's are in the right order upon checkpoint creation

When we are assigning sequence numbers we need to make sure that
if a checkpoint start message is added to the checkpoint manager
then we need to also make sure that we assign a sequence number
to the checkpoint start message before the mutation.

Change-Id: I9529d6866573e4fcf47d419bd71c3a32d75f4bf3
Reviewed-on: http://review.couchbase.org/31462
Reviewed-by: David Liao <david.liao@couchbase.com>
Tested-by: David Liao <david.liao@couchbase.com>