Allow to run async code in a loop that already running

[uriyyo]

Refactor asyncio integration to allow run async code in currenly running loop.

I've been working on a library called async-eval and PyCharm plugin built on top of it pycharm-evaluate-async-code . Both libraries have existed for a while and have proved to work for many users (me included). Basically, they patch pydevd code to use exectly same approach as in this PR.

The main idea is to manually trigger _loop_once on currenlty running loop until future won't be done, for non-running eventloop old approach will be used.

Thanks, and let me know what do you think 🙏