"Flatten" the tasks in the light client

[tomaka]

cc #345

The light client spawns various tasks that are run in the background.
It also creates two FuturesUnordered (one in the networking service and one in the JSON-RPC service), pushes tasks to it, and then spawns a background task that runs this FuturesUnordered.
In other words, there's a "hierarchy" of tasks: the tasks spawned in the background, and the tasks that are run by tasks spawned in the background.

The advantage of doing so and the reason why this was done so is that the "second level" of the "hierarchy" doesn't have the same priority as the "first layer". The first layer contains important tasks, and the second layer the less important ones.

Unfortunately, the FuturesUnordered doesn't work well when it comes to yielding. It's only after tasks have yielded twice that the FuturesUnordered yields. Because the wasm node implements background tasks with a FuturesUnordered as well, this means that all tasks have to yield a total of 4 times in order to "actually" yield to the browser/NodeJS/Deno.
At the moment, yielding is implemented by sleeping twice, for exactly this reason.

See rust-lang/futures-rs#2053 for a discussion about this.

In order to solve that problem, this PR removes the layers of hierarchy. This means that all tasks now have the same execution priority, which isn't great in the absolute but in practice isn't a problem. If we need to restore priority levels, I think a better solution is to add a parameter when spawning a background task indicating the priority.

[github-actions]

twiggy diff report

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

---

 Delta Bytes │ Item
─────────────┼─────────────────────────────────────────────────────────────────────────────────────────────────────────────────
      +10359 ┊ smoldot_light::network_service::tasks::connection_task::{{closure}}::h89ed143d320ef28a
      -10359 ┊ smoldot_light::network_service::tasks::connection_task::{{closure}}::h9b6ac1a752be546a
       -3597 ┊ smoldot_light::sync_service::SyncService<TPlat>::storage_query::{{closure}}::ha7241b4cdca61fd9
       +3597 ┊ smoldot_light::sync_service::SyncService<TPlat>::storage_query::{{closure}}::ha9f1c70c7ab5574f
       -3362 ┊ smoldot_light::sync_service::SyncService<TPlat>::storage_query::{{closure}}::h4bb1b97f70ee39d5
       +3362 ┊ smoldot_light::sync_service::SyncService<TPlat>::storage_query::{{closure}}::hd89b1c34f36f3308
       +2573 ┊ smoldot_light::network_service::NetworkService<TPlat>::blocks_request::{{closure}}::hcbe957a65a360970
       -2573 ┊ smoldot_light::network_service::NetworkService<TPlat>::blocks_request::{{closure}}::hcdba85108b71db3e
       +2380 ┊ smoldot::json_rpc::requests_subscriptions::SubscriptionStart<TSubMsg>::start::{{closure}}::h4eeea666fa593a18
       -2380 ┊ smoldot::json_rpc::requests_subscriptions::SubscriptionStart<TSubMsg>::start::{{closure}}::h859fcc5f4226b4ed
       +2138 ┊ smoldot_light::json_rpc_service::background::start::haf715da9ef48a2c7
       +2125 ┊ futures_util::abortable::Abortable<T>::try_poll::h05395081a639f6a5
       -2072 ┊ smoldot_light::json_rpc_service::background::Background<TPlat>::run::{{closure}}::{{closure}}::ha705609b9a1ebe2c
       -1842 ┊ futures_util::abortable::Abortable<T>::try_poll::h1de2dfd766b71ae6
       +1842 ┊ futures_util::abortable::Abortable<T>::try_poll::hfa0b62c336eb5bb3
       +1629 ┊ futures_util::abortable::Abortable<T>::try_poll::hc69260a39906c13b
       -1629 ┊ futures_util::abortable::Abortable<T>::try_poll::hd19a257316c29fcd
       -1267 ┊ smoldot_light::sync_service::SyncService<TPlat>::block_query_unknown_number::{{closure}}::h151ba8148c0b56ee
       +1267 ┊ smoldot_light::sync_service::SyncService<TPlat>::block_query_unknown_number::{{closure}}::hc0167e83a79eebd7
       -1250 ┊ <core::pin::Pin<P> as core::future::future::Future>::poll::h70f0a8dfc0e04a15
       -3241 ┊ ... and 350 more.
       -7173 ┊ Σ [370 Total Rows]