Process incoming foreground messages while subscribing

[tomaka]

Calling relay_chain_sync.subscribe_all(...).await might take a long time (as it waits for the runtime of the chain to have been downloaded), during which messages coming from the public API of the sync service just pile up and aren't answered.
This PR fixes this by answering messages while we wait for the subscription to happen.

The consequence is that a parachain initialization now finishes quickly, while before it was waiting for its relay chain initialization to finish.
It also means that we now properly clean up parachains if we remove them, even if their relay chain never downloads its runtime. Before, the clean up didn't happen because the clean up never happens before initialization is complete.

I'm not really happy with the code duplication here, and would perform a refactoring that moves local variables into a struct, and adds methods to that struct, similar to standalone.rs.

[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
─────────────┼───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
      +14675 ┊ smoldot_light::sync_service::parachain::start_parachain::{{closure}}::h8061ad0acae26644
      -13359 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::hd71ed9b59fff1e59
        -438 ┊ core::ptr::drop_in_place<core::future::from_generator::GenFuture<smoldot_light::sync_service::parachain::start_parachain<smoldot_light_wasm::platform::Platform>::{{closure}}>>::h8f9ceeff87e013f0
        +438 ┊ core::ptr::drop_in_place<core::future::from_generator::GenFuture<smoldot_light::sync_service::parachain::start_parachain<smoldot_light_wasm::platform::Platform>::{{closure}}>>::hac18bc409a8df5c0
        +402 ┊ core::ops::function::impls::<impl core::ops::function::FnMut<A> for &mut F>::call_mut::h305b9a50768ad49b
        -402 ┊ core::ops::function::impls::<impl core::ops::function::FnMut<A> for &mut F>::call_mut::h9d82817830bade80
        -392 ┊ itertools::Itertools::join::h012c3b637f14ff2b
        -392 ┊ itertools::Itertools::join::h04050778a178e584
        -392 ┊ itertools::Itertools::join::h359d21405d4c8abf
        +392 ┊ itertools::Itertools::join::h4c8141177e0d3245
        +392 ┊ itertools::Itertools::join::h6115c78fa4ead132
        +392 ┊ itertools::Itertools::join::hc5bc1b70b523b48d
        -376 ┊ <futures_util::future::poll_fn::PollFn<F> as core::future::future::Future>::poll::h549c667bade04da5
        +376 ┊ <futures_util::future::poll_fn::PollFn<F> as core::future::future::Future>::poll::haa64cf9c3e72464d
        +283 ┊ <futures_util::future::poll_fn::PollFn<F> as core::future::future::Future>::poll::h9bded82f4b1d9b32
        -270 ┊ <alloc::vec::Vec<T> as alloc::vec::spec_from_iter_nested::SpecFromIterNested<T,I>>::from_iter::h8a50b8701d2c5745
        +270 ┊ <alloc::vec::Vec<T> as alloc::vec::spec_from_iter_nested::SpecFromIterNested<T,I>>::from_iter::ha79d8df8a2479650
        +270 ┊ <alloc::vec::Vec<T> as alloc::vec::spec_from_iter_nested::SpecFromIterNested<T,I>>::from_iter::hf0b711e0b28d8239
        -259 ┊ <alloc::vec::Vec<T> as alloc::vec::spec_from_iter_nested::SpecFromIterNested<T,I>>::from_iter::h47c114ba723148ca
        +259 ┊ <alloc::vec::Vec<T> as alloc::vec::spec_from_iter_nested::SpecFromIterNested<T,I>>::from_iter::hd7049b7d9f6bc15f
       +3156 ┊ ... and 209 more.
       +9663 ┊ Σ [229 Total Rows]