Map AsyncIterator (& coroutines?) asynchronously

[juanmirocks]

(Not currently needed for the scheduling, which already works async, but check_all_once runs the checks in deterministic sequence)

Mapping AsyncIterators is not supported by Python natively yet.

Try 3rd-party libs:

• https://asyncstdlib.readthedocs.io/en/stable/index.html
• https://aiostream.readthedocs.io/en/stable/

Especially, try to use for code in postgres's sockets & checks_runner.py