Security: Send notfound inv items to the inv collector, Credit: Equilibrium

[teor2345]

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

Zebra currently ignores notfound responses from peers.

1. To avoid duplicate requests for notfound items, if a peer has recently told us that item was not found, we should redirect or reject that item request.
2. We should also synthesise notfound inv entries for peers that didn't respond to a request, even if they don't send a notfound message themselves.

This helps prevent request/response loops.

A similar issue was reported by Niklas Long of Equilibrium.

Describe the solution you'd like

• Send notfound messages to the inv collector
• Synthesise notfound messages to the inv collector when peers don't find an item, if the peer doesn't actually send a notfound message
• Synthesise notfound messages to the inv collector when the peer request times out, or is dropped for some other reason (use the Drop impl)
  • Document this behaviour in zebra_network::Request, and also document that requests for multiple items wait for data or notfound for all of them (or wait for an obviously unrelated response)
• When a Client sends a request for an item:
  1. direct it to a peer that has the item (inv collector)
  2. direct it to a peer that hasn't recently returned notfound for the item
  3. reject the request and return an error

Describe alternatives you've considered

This doesn't seem to be causing any issues right now, so we might not need to do this fix.

Dependencies

This change might be easier after we do #1973.