Free lease queue allocator

The Free lease queue allocator populates leases at the startup or reconfiguration in the server's memory. It installs callbacks in LeaseMgr using #2764 (closed). When a DHCP request comes in, it picks the next free lease for a subnet and offers it to the client. When the client requests the lease and the lease is allocated, the callbacks are invoked causing the lease removal from the populated list. Similarly, when the lease is deleted or reclaimed, the lease is put back into the queue.

This ticket doesn't necessarily contain optimizations to inherit populated leases across reconfigurations. It means that the leases will be populated every time, the server is reconfigured. But, that's fine for the first step. We will address it in another ticket.

Also see: https://gitlab.isc.org/isc-projects/kea/-/wikis/Designs/free-lease-queues-design