Threadpool should have an API for waiting for spawned tasks

[withoutboats]

If you spawn a bunch of tasks on the threadpool from the main thread, and then the main thread concludes, the program will terminate before the threadpool's tasks have finished. Ideally, there'd be an easy way to ensure that all the spawned tasks conclude before the program ends.

[Nemo157]

I thought that’s what ThreadPool::run did, but it appears it’s only LocalPool::run that behaves this way. Would be good to have consistent naming and behaviour between pools as well.

[Diggsey]

It's pretty hard to use thread-pool without some way to wait for its tasks to complete.

[alexanderlinne]

Is there anyone working on this? I'd be happy to implement a join function