Simplify asyncio and server list

[alexrudd2]

Note: this code is very confusing to me, so @janiversen please review carefully.

Solves another 5 mypy errors, and speeds up the Window tests.

Instancing the class _serverList from its own method (1) is... strange? (2) is unnecessary to use the methods, which are all @classmethod (3) is unnecessary to keep track of the event loop, since every server class already has self.loop ==> Use the class methods directly

The methods shutdown() and server_close() are essentially redundant. ==> combine

There's no use for asyncio.wait() directly after an await. ==> remove

time.sleep() is inefficient, as described in https://github.com/pymodbus-dev/pymodbus/issues/1992#issue-2128311471) ==> replace with waiting+timeout for the Future