Host reservation based classification should influence lease allocation

It is possible to specify per-client client classes within the host reservations. These classes, however, are used after the client has already got a lease and merely influence which DHCP options are given to the client. This is fine for the case of non-global host reservations which rely on the ID of the selected subnet.

In case of the global reservations there is no dependency on the subnet ID, so the classes from reservations could actually be assigned earlier and that way used to influence lease allocation or subnet selection.

There are some real life cases when such need is present. Currently the only host reservation dependent classes available prior to lease allocation are KNOWN/UNKNOWN built-ins. They can be used for subnet selection but such approach has number of limitations.