Only send transactions to peers with open substream

[tomaka]

Fix #2680

The code in network_service was wrong and was attempting to send a transaction to every peer we have a connection with, instead of every peer we have opened a transactions notifications substream with.

[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
─────────────┼─────────────────────────────────────────────────────────────────────────────────────────────────────────────────
        +223 ┊ <alloc::vec::Vec<T> as alloc::vec::spec_from_iter_nested::SpecFromIterNested<T,I>>::from_iter::he836e1c777861a19
        +151 ┊ alloc::vec::Vec<T,A>::extend_desugared::h05985a9fee4fc06f
        +128 ┊ <core::iter::adapters::cloned::Cloned<I> as core::iter::traits::iterator::Iterator>::next::ha8122863ac82687b
        +109 ┊ smoldot::libp2p::peers::Peers<TConn,TNow>::opened_out_notifications::h53f0bb2f61c9bad0
         -95 ┊ core::iter::traits::iterator::Iterator::collect::hef4d1423c37a57e5
         +56 ┊ <alloc::vec::Vec<T,A> as alloc::vec::spec_extend::SpecExtend<T,I>>::spec_extend::h366f72fb1c4a94b2
         +56 ┊ <alloc::vec::Vec<T> as alloc::vec::spec_from_iter::SpecFromIter<T,I>>::from_iter::h952e1fbd8c711e61
         +30 ┊ core::ops::function::impls::<impl core::ops::function::FnOnce<A> for &mut F>::call_once::h0ad6b1ef2a811088
         -27 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::hba94edd9867ebb6a
         +22 ┊ smoldot::network::service::ChainNetwork<TNow>::opened_transactions_substream::h401cf4ef955f35cc
         +16 ┊ data[0]
         +10 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::h61f7287cd92add74
       +1605 ┊ Σ [12 Total Rows]