Hang on shutdown

[Flozza]

In some cases, the client hangs on shutdown. The way to reproduce is just retry a few times, letting it run (or not, varying). When it hangs, it will hang after the flag collector is shut down but before the submitter is finished, so likely it hangs on joining a submitter.

It should be examined what hangs there and it should be fixed...

[Flozza]

We use asyncio now