Messages and message encoding has nothing
to do with the underlying authenticated
communications framework. Separate these
two.

Protocol was actually made of two sub-protocols.
  * a lower-half handling authentication, encryption,
key exchange, and request/response. This is now called
Transport.
  * an upper-half handling DHT messages. This is still
called Protocol.

Separation of these two reduces dependencies and simplifies
modifications to the protocol.
Signed-off-by: Csaba Kiraly <[email protected]>

Note that the test does not compile, but it was
also not compiling before

Signed-off-by: Csaba Kiraly <[email protected]>

- interface between Transport and Protocol is at the encoded Message
level

Node already has the address, so it does not make sense to
pass it as a separate parameter.

This is a request part of a Request/Response, generating
also a reques ID. So call it what it is.

This completes the Request/Response semantics.

It is better to rely on protocol.nim to do all the encoding,
thus cleaning up dependencies.