Test translation for zebra-network::{Request, Response} protocol

[hdevalence]

Is your feature request related to a problem? Please describe.

We need to complete the translation layer between Bitcoin messages and zebra_network::{Request, Response} pairs.

Describe the solution you'd like

• Each Message variant should generate a zebra_network::Request, or be explicitly ignored. Each zebra_network::Response variant should generate a Message, or in the case of Response::Nil, generate no message. (Done in Message/Request translation #1075)

• Refactor the Zebra requests and response state into distinct request/response pairs (Refactor zebra-network Bitcoin to Zebra protocol translation layer #1515)

• The translation should be tested by connecting two connection state machines and seeing that they translate back and forth correctly.

• The translation should be tested against zcashd, as much as possible

Some of this work was implemented a long time ago, and some of it was implemented in #1016. The rest is to be completed.

Prerequisite for actually handling the requests in #889.

[teor2345]

We have implemented the network protocol, and have partial tests between two zebra-network instances.

We should open more specific tickets for missing tests, or just do them as we modify the protocol.