Implement simple DHCP Leasequery functionality (per RFC4388 and RFC5007)

Kea DHCP (up to and including 1.7.1) does not currently support Leasequery in any form.

In Support ticket #15003 a specific use case is detailed - and it is likely that there are many more users of Kea DHCP who would like to avail themselves of this functionality.

Here is one use case for this:

We host an HFC (Hybrid Fiber coax) infrastructure providing internet service over DOCSIS 3.0 and 3.1. Our core infrastructure is based on Cisco CMTS. We have configured our CMTS such that when a device previously unknown in the network generates/receives IP traffic without an associated ARP entry in CMTS, the CMTS performs a DHCP leasequery to our current DHCP server. The aim is to determine whether the CPE has been given an IP from our DHCP server (versus being statically configured and connected). The CMTS uses the leased IP and the MAC address associated with it that is returned by the DHCP server.

This use case requires simple leasequery functionality.

See also #979 (closed) for bulk leasequery functionality.