Fix starting requests on shutting down connections

[tomaka]

Fixes a TODO left in the code. I'm slowing catching up on networking TODOs.

Fix #2777
Fix #2841

This PR changes has_established_connection to return false if all the connections are shutting down.
To clear up any ambiguity, I've also renamed it to can_start_requests, because determining whether we can send requests is the reason why this function is called.

[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
─────────────┼─────────────────────────────────────────────────────────────────────────────────────────────────────────────
        +104 ┊ <core::iter::adapters::map::Map<I,F> as core::iter::traits::iterator::Iterator>::try_fold::h903a2ef4d4916159
         -61 ┊ smoldot::network::service::ChainNetwork<TNow>::has_established_connection::h0a2ba2c3ffa39079
         +60 ┊ smoldot::libp2p::peers::Peers<TConn,TNow>::can_start_requests::h21daa631f106677d
         +32 ┊ <either::Either<L,R> as core::iter::traits::iterator::Iterator>::any::hd5950a8403797e19
          +6 ┊ type[23]: (i32, i64) -> i32
          -6 ┊ type[23]: (i64, i32) -> i64
          +6 ┊ type[24]: (i64, i32) -> i64
          -6 ┊ type[25]: (i32, i64) -> i32
          -5 ┊ type[24]: (i32, i64) -> nil
          +5 ┊ type[25]: (i32, i64) -> nil
        +386 ┊ Σ [10 Total Rows]