rework `shutdown zipper` to give better determinism, performance

[Groostav]

The current shutdown zipper attempts to provide deterministic behaviour in:

• exitCode.join()
• aggregateChannel().consumeEach{}
• runningProcess.join()

but the problem seems to be that resuming coroutines is intrinsically tough to make deterministic. There is both a knowledge-problem and a tech problem here.

1. determine if there is a sufficient way to collect the above calls and dispatch them in some kind of known order without a messy blocking system
2. replace shutdown zipper with something that doesn't use locks.