Automatically invalidate preselected peers

[teor2345]

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

In zebra_network::peer_set::set, we use an integer index to track the preselected peer index. This is a Rust anti-pattern, replacing Rust's ownership constraints with manual ownership management. (It's still safe, but if we get it wrong, it causes bugs.)

Describe the solution you'd like

Use tower::steer: https://docs.rs/tower/0.4.7/tower/steer/index.html

Describe alternatives you've considered

Instead of storing the ready peers in a Vec, store them in a HashMap<Discover::Key>. (The key is the remote SocketAddr of the peer's TCP connection.)

Continue to use manual index tracking, which has no unit tests. It is hard to maintain, and prone to bugs.

[teor2345]

Marking this as a low priority, because the code seems to work right now, and it hasn't panicked since the most recent fixes.

[teor2345]

This was fixed by PR #3090.