Add 1:1 Scheduling Mode

[eholk]

Add a way to make each task run in its own thread.

[brson]

I'm planning on reintroducing the domain concept, where our current set of schedulers would be something like a 'generic' domain that just does it's best to schedule across cores. Then we would also have a thread domain, which is just a single-threaded scheduler. Then we could have a 1:1 domain where all tasks get their own thread. Possibly we could have some mechanism to pick the scheduler that main spawns into.

[brson]

The task API surfaces this concept now, but it's still not implemented in the runtime.

[bstrie]

@brson, is this still relevant with the upcoming scheduler?

[huonw]

Triage visit; echoing @bstrie's question: @brson, is this still relevant with the new scheduler?

[toddaaro]

Done in PR #8221