Resolve "Modification of handling of global reservations with IP addresses set"

Closes #2631 (closed) Closes #2632 (closed)

As discussed on the support ticket and in #2632 (closed), this MR applies a revised version of the customer's patch such that:

Kea will ensure that globally reserved addresses fall within the selected subnet before offering a lease for them. If the address falls outside the initially selected subnet Kea will check the address against the subnet's siblings if in a shared-network, and select the first sibling that contains the address. If no containing subnet is found Kea falls back to dynamic allocation.

I opted to not make the behavior configurable as the default (broken) behavior is something the ARM cautioned people against. The broader topic of global hosts with multiple fixed addresses is outside the scope of this MR.