MB-7018: fail if the user tries incr with cas

Change-Id: Iedaeffc65b507d5655dd257b9dc1b67fc9effdbb
Reviewed-on: http://review.couchbase.org/21971
Reviewed-by: Dustin Sallings <dustin@spy.net>
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>