fix: there should be only one exit event

[metakot]

So AppStatus.should_exit_event got rewrited in listen_for_exit_signal with every new connection being made and old connections remain in connected state when server stops.
This fixes that.

[sysid]

Nicely spotted @metakot, thank you !

[sysid]

Problem is: Parametrized tests are failing now with:
RuntimeError: <asyncio.locks.Event object at 0x1046a0a30 [unset]> is bound to a different event loop

I fixed it.