MB-7580: Make sure mutation number is always increasing

Change-Id: Ic079c9b78f3f4bd9cd88b0605853e80120b72ced
Reviewed-on: http://review.couchbase.org/24147
Reviewed-by: Jin Lim <jin@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>