Allow jobs to be defined as coroutines

[RealOrangeOne]

For some libraries, it's necessary to use async code, even if it's in a single process where the async benefits don't apply.

This transparently calls a job function as a coroutine if it is one, allow jobs defined async def my_job(job): to "just work" ™️.

Note that this doesn't use a shared event loop, it simply starts one at execution time.

[j4mie]

This PR is a bit stale and I think probably not needed - if you do need to run some async code in a job or hook, you can just handle the async-to-sync stuff yourself.