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/exchange_rate.py

@@ -148,7 +148,7 @@ class ExchangeBase(Logger):
         if h is None:
             h = self.read_historical_rates(ccy, cache_dir)
         if h is None or h['timestamp'] < time.time() - 24*3600:
-            asyncio.get_event_loop().create_task(self.get_historical_rates_safe(ccy, cache_dir))
+            util.get_asyncio_loop().create_task(self.get_historical_rates_safe(ccy, cache_dir))
 
     def history_ccys(self) -> Sequence[str]:
         return []
@@ -471,7 +471,7 @@ def get_exchanges_and_currencies():
                 for name, klass in exchanges.items():
                     exchange = klass(None, None)
                     await group.spawn(get_currencies_safe(name, exchange))
-    loop = asyncio.get_event_loop()
+    loop = util.get_asyncio_loop()
     try:
         loop.run_until_complete(query_all_exchanges_for_their_ccys_over_network())
     except Exception as e: