refactor spawn API

[niklasad1]

I would want the spawn and spawn_blocking to return a spawn handle or something equivalent and as substrate uses tokio quite extensively looks reasonable to return tokio::task::JoinHandle instead.

Needed by #13992

No real code changes rather than tests and benchmarks

[paritytech-cicd-pr]

The CI pipeline was cancelled due to failure one of the required jobs.
Job name: test-linux-stable
Logs: https://gitlab.parity.io/parity/mirrors/substrate/-/jobs/2744791

[bkchr]

A spawn handle is just:

let (rx, tx) = oneshot::channel();

spawner.spawn(async move { do_something.await(); let _ = tx.send(()); });

rx.await();

[niklasad1]

yeah, but I didn't to make code so verbose with oneshot's when tokio spawn already has it..

I can add the oneshot stuff and you can complain in my other pr :D

[bkchr]

I can add the oneshot stuff and you can complain in my other pr :D

Let's try that for now :P

yeah, but I didn't to make code so verbose with oneshot's when tokio spawn already has it..

If we add the JoinHandle, I would still make it generic. Aka something that wraps a Box<Future>. I know that we are currently using Tokio, but it doesn't leak that much to the internals right now.

[niklasad1]

Fair enough

We could add some such as impl Future<Item = Result<(), ()>>` as return handle but yeah closing this