Fix flaws in the API of peers.rs

[tomaka]

Close #2370

This PR removes weirdnesses in the implementation of peers.rs by improving its API. There is now a StartShutdown event similar to collection.rs, after which it is invalid to start requests. After this event has been generated, all open substreams will be closed.

See #2369 for more context.

I think I've preserved the same behavior in service.rs as before this PR, but I'm not completely sure of that. One exception is that we now properly clean up k-buckets when a connection was still handshaking is shut down.
I will improve service.rs as a follow-up to this PR.

[mergify]

Automatically approving tomaka's pull requests. This auto-approval will be removed once more maintainers are active.

[github-actions]

twiggy diff report

Difference in .wasm size before and after this pull request.

---

 Delta Bytes │ Item
─────────────┼─────────────────────────────────────────────────────────────────────────────────────────────────────────
       -4491 ┊ smoldot::libp2p::peers::Peers<TConn,TNow>::next_event::h666cfc906727b7e9
       +4313 ┊ smoldot::libp2p::peers::Peers<TConn,TNow>::next_event::h7668105a2b015c60
       -3636 ┊ smoldot::libp2p::collection::Network<TConn,TNow>::next_event::hd3decd1b94c9165f
       +3607 ┊ smoldot::libp2p::collection::Network<TConn,TNow>::next_event::hcbcf200208b538da
       +1071 ┊ hashbrown::raw::RawTable<T,A>::reserve_rehash::hecdda9ec2788a2a6
        -974 ┊ hashbrown::raw::RawTable<T,A>::reserve_rehash::h8853276c0fe71d92
        +877 ┊ smoldot::libp2p::collection::Network<TConn,TNow>::new::hc44c39a7ff318bac
        -877 ┊ smoldot::libp2p::collection::Network<TConn,TNow>::new::he7ccf4c8bb989fd3
        -767 ┊ smoldot::libp2p::peers::Peers<TConn,TNow>::new::h0321a30cb2d79fa0
        +761 ┊ smoldot::libp2p::peers::Peers<TConn,TNow>::unfulfilled_desired_peers::h8b26c0dcc0dcf53f
        -761 ┊ smoldot::libp2p::peers::Peers<TConn,TNow>::unfulfilled_desired_peers::hc03d905eb3e219cb
        +599 ┊ smoldot::libp2p::peers::Peers<TConn,TNow>::new::h032b5cd57b39b6c2
        +585 ┊ smoldot::libp2p::peers::Peers<TConn,TNow>::set_peer_notifications_out_desired::h492018cea696be50
        -585 ┊ smoldot::libp2p::peers::Peers<TConn,TNow>::set_peer_notifications_out_desired::h9e962a98fcaf6513
        +504 ┊ <core::iter::adapters::map::Map<I,F> as core::iter::traits::iterator::Iterator>::fold::h46d014401c5ba2ac
        -504 ┊ <core::iter::adapters::map::Map<I,F> as core::iter::traits::iterator::Iterator>::fold::h927616064ec0d40c
        +495 ┊ smoldot::libp2p::peers::Peers<TConn,TNow>::try_clean_up_peer::h5d0a36ebe7ac7d13
        -495 ┊ smoldot::libp2p::peers::Peers<TConn,TNow>::try_clean_up_peer::h75d4244451d0d17d
        -438 ┊ smoldot::libp2p::collection::Network<TConn,TNow>::insert_single_stream::heee6ce585e5638cf
        -435 ┊ <core::iter::adapters::map::Map<I,F> as core::iter::traits::iterator::Iterator>::fold::h15c902de2910dc7d
       -2520 ┊ ... and 304 more.
       -5734 ┊ Σ [324 Total Rows]