Implement the ISC DHCP stash-agent-options in Kea

See #218 for details but the idea is to handle v4 renew/rebind queries as they went thought a relay. As far as I can remember the idea is to use the hint (requested or client address) from the query to look up the lease and extract the RAI from its user context. Requires some design but on the paper this should do the job...

Note this is for v4 only because v6 requires a specific setup on both the client and the server for direct unicast queries so the problem never occurs in the real world.