Simplify implementation of multiple background jobs

[rrthomas]

See the simplify branch, which currently over-simplifies, and reverts to running only one job at a time.

Presumably the problem is that the subprocess calls do not yield, and so do not allow for multitasking. So try going back to asyncio subprocesses.

[rrthomas]

The problem with the code that I want to fix is the CommandExpansion type's first member, which can be an (immediate) bytes or an async Command. It should just be one or the other.