Refactor zebra-network Bitcoin to Zebra protocol translation layer

[teor2345]

Motivation

The Bitcoin to Zebra protocol translation layer is a bit of a mess, and a source of subtle bugs:
https://github.com/ZcashFoundation/zebra/blob/main/zebra-network/src/peer/connection.rs

It contains a lot of undocumented state, assumptions and invariants. And it's unclear if these assumptions match the zcashd implementation.

We should refactor the protocol translation into cleaner request/response pairs. We should then test those requests and responses against Zebra and zcashd (#1048).

Current Implementation

There is a summary of the current implementation here:
https://docs.google.com/document/d/1KgPl3r_Pp8AUM0KvDnBv7LK0vRMvxVjM9ooIwr2mB7U/edit#

Design Questions

Handling Incomplete Peer Responses

• How should we handle incomplete block or transaction responses from peers?
  • done in Make Zebra send notfound, and use received notfound to finish requests #2726
• Should we patch zcashd to return NotFound for blocks?
• Should we make the connection timeouts shorter, or fill in missing timeouts?
  • done in other tickets

Internal Protocol

• Should incomplete block or transaction responses include a list of missing items?
  • done in Make Zebra send notfound, and use received notfound to finish requests #2726
• Is it really an error if we ask for a block or transaction hash, but the peer doesn't know it? (Not an error)
  Should we close the connection if there is a missing item? (No)
  • send the responses we've received so far, and try to process the extra response as a request
  • done in Security: Stop disconnecting from nodes that send unexpected messages, to prevent disconnection attacks, Credit: Equilibrium #2107
• Should we fake a NotFound response for a missing item, even if the peer doesn't send a NotFound?
  • done in Make Zebra send notfound, and use received notfound to finish requests #2726

[teor2345]

Actually, the open design questions are answered by other tickets, but the task still needs to be done.

[teor2345]

This is getting done gradually as we change the code.