Implement Bulk DHCPv6 Leasequery functionality (per RFC6926 and RFC5460)

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

The basic leasequery (RFC 4388, RFC 5007) was added in 1.8.0. It supports both v4 and v6.

The bulk leasequery (RFC 6926, RFC 5460) remains not supported as of 2.1.7.

In Support ticket #15003 a specific use case is detailed, primarily for simple leasequery, but as a backup/recovery mechanism, bulk leasequery would also be very useful.

Feature request #978 (closed) has been opened for simple Leasequery. This is the use case for bulk Leasequery:

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.

The above requirement is satisfied by simple leasequery, but it would be good also to be able to make use of DHCPv6 bulk-lease-query which, from the RFC and from Cisco's recommendation allows the CMTS to perform a bulk request for leases when there is a network-wide outage, thus optimizing the boot up process. The aim is to fill in the mac address table within the CMTS from what can be learned from the DHCP server(s).