[#1428] Configure DHCP servers to accept and use multiple reservations for the same IP address

This MR should introduce new global configuration parameter ip-reservation-unique with a default value true. When set to false the servers will accept multiple reservations for the same IP address. Obviously, that has to correspond with the backends in use. If any of the backends does not support it, this setting should be refused. When in use, the allocation engine should also assume that multiple reservations can be returned for the given IP address.

#huge-sorry