Implement TKEY exchange

One of the operations required by the GSS-TSIG hook is the ability to establish security context with the DNS server using TKEY protocol. We have the TKEY RR type added in #1880 (closed), we now need to start using it.

There is an example implementation available in #1935 (closed), which is a kind of an experiment. The TKEY exchange part of it should be turned into production code in GSS-TSIG hook.