Move _set_running_loop up the stack

[dandavison]

This is a draft PR, investigating where we can / should set/unset the running loop.

Currently it's set/unset on entry/exit of _run_once(). Is the purpose of that to prevent any execution of workflow coroutines at any other time during activate()?

If so: CI is green when we do the setting/unsetting at the top level in activate(): it would be great to have a failing test demonstrating the necessity of confining the execution to _run_once().

[cretz]

I do not have a strong preference here. It was originally done because I expected all real workflow-context work to happen inside a task. I didn't want any code not running in an asyncio task to have a loop. In CPython they set the loop on the thread on asyncio.run or loop.run_forever or whatever which makes sense to me. If Python had a loop.step or something, I'd expect it to set and unset the running loop within that too, and _run_once is meant to just that missing Python library method.