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/daemon.py

@@ -124,7 +124,7 @@ def request(config: SimpleConfig, endpoint, args=(), timeout=60):
         rpc_user, rpc_password = get_rpc_credentials(config)
         server_url = 'http://%s:%d' % (host, port)
         auth = aiohttp.BasicAuth(login=rpc_user, password=rpc_password)
-        loop = asyncio.get_event_loop()
+        loop = util.get_asyncio_loop()
         async def request_coroutine():
             if socktype == 'unix':
                 connector = aiohttp.UnixConnector(path=path)
@@ -467,7 +467,7 @@ class Daemon(Logger):
         if 'wallet_path' in config.cmdline_options:
             self.logger.warning("Ignoring parameter 'wallet_path' for daemon. "
                                 "Use the load_wallet command instead.")
-        self.asyncio_loop = asyncio.get_event_loop()
+        self.asyncio_loop = util.get_asyncio_loop()
         self.network = None
         if not config.get('offline'):
             self.network = Network(config, daemon=self)