allow per type record counts to be specified in update-policy

Allow per type limits to be specified as part of a update policy. This will allow ISP's to specify practical limits.

e.g. Clients could be allowed to add a single PTR record for their machine.

zone "in-addr.arpa" {
	type master;
	file "x";
	update-policy {
		grant * tcp-self . ptr(1);
	};
};

closes #1657 (closed)