async: Is it possible to have a background task?

[alexcrichton]

I believe the answer is "no", but I wanted to confirm. I believe this is likely used by libraries that do some sort of connection pooling or similar where a background task might be performing maintenance on behalf of the pool or similar. This is somewhat tangentially related to #459 as well.

[dicej]

I think that's what this PR is about.

[alexcrichton]

Ah I've been a bit oblivious of everything start related since it's not implemented in Wasmtime and probably won't be for some time, but despite that I also think that may not necessarily work for the use cases I'm thinking of where it would require restructuring libraries to expose pools/background tasks where they're probably currently internal today. We can talk more about this in the sync next week though