asyncio: stop using get_event_loop(). introduce ~singleton loop.

asyncio.get_event_loop() became deprecated in python3.10. (see https://github.com/python/cpython/issues/83710)
```
.../electrum/electrum/daemon.py:470: DeprecationWarning: There is no current event loop
  self.asyncio_loop = asyncio.get_event_loop()
.../electrum/electrum/network.py:276: DeprecationWarning: There is no current event loop
  self.asyncio_loop = asyncio.get_event_loop()
```
Also, according to that thread, "set_event_loop() [... is] not deprecated by oversight".
So, we stop using get_event_loop() and set_event_loop() in our own code.
Note that libraries we use (such as the stdlib for python <3.10), might call get_event_loop,
which then relies on us having called set_event_loop e.g. for the GUI thread. To work around
this, a custom event loop policy providing a get_event_loop implementation is used.

Previously, we have been using a single asyncio event loop, created with
util.create_and_start_event_loop, and code in many places got a reference to this loop
using asyncio.get_event_loop().
Now, we still use a single asyncio event loop, but it is now stored as a global in
util._asyncio_event_loop (access with util.get_asyncio_loop()).

I believe these changes also fix https://github.com/spesmilo/electrum/issues/5376

electrum/commands.py

@@ -187,7 +187,7 @@ class Commands:
                 kwargs.pop('wallet')
 
         coro = f(*args, **kwargs)
-        fut = asyncio.run_coroutine_threadsafe(coro, asyncio.get_event_loop())
+        fut = asyncio.run_coroutine_threadsafe(coro, util.get_asyncio_loop())
         result = fut.result()
 
         if self._callback: