[#2719] ip address re-offer cooldown V4

Closes #2719 (closed)

Adds a parameter to kea-dhcp4. offer-lifetime, that can be specified at the global, shared-network, subnet, and class scopes. When greater than zero, it more or less negates fake_allocation flag. Please see the new ARM section for details.

This MR only applies to kea-dhcp4, whether or not we want similar behavior for v6 is TBD. V6 Users can achieve a better result with rapid commit assuming their clients support it.