Ability to split a JoinSet into spawn and join halves

[slonkazoid]

Is your feature request related to a problem? Please describe.

JoinSet requires a mutable reference to spawn or join tasks. This means you cannot have 2 concurrent tasks appending tasks and joining them.
This may be required for example when you have to spawn multiple millions of tasks so you use a semaphore and acquire a ticket before you spawn a new task.

Describe the solution you'd like

A simple fn split(self) -> (SpawnerHalf, JoinerHalf) method like this:

let (mut spawner, mut joiner) = JoinSet::new().split();

let task_a = tokio::task::spawn(async move {
    spawner.spawn(async { 42i32 });
});

while let Some(joined) = joiner.join_next().await {
  println!("the meaning of life is: {}", joined.unwrap());
}

task_a.await.unwrap();

Describe alternatives you've considered

• Mutex. Deadlocks with no obvious way to fix it.
• Making a JoinSet equivalent that takes a &self so it can be put in an Arc and cloned around.
• Passing my own waker into join_next().

[Darksonn]

This is not a use-case that JoinSet supports. Some alternatives:

1. If you don't need return values, then consider the TaskTracker.
2. If you need return values, then consider sending them using a mpsc channel.