aiorpcx: adapt TaskGroup._add_task monkey-patch to upstream fix

- upstream fix is at kyuupichan/aiorpcX@a0be71c ,
  but it's not yet released
- prev monkey-patch was causing issues if used with aiorpcx that already has the upstream fix:

```
INFO:SessionManager:TCP server listening on 127.0.0.1:51001
INFO:SessionManager:SSL server listening on 127.0.0.1:51002
INFO:PeerManager:peer discovery: ON
INFO:PeerManager:announce ourself: True
INFO:PeerManager:my clearnet self: None
INFO:PeerManager:force use of proxy: False
INFO:PeerManager:beginning peer discovery...
INFO:PeerManager:trying to detect proxy on "localhost" ports [9050, 9150, 1080]
Exception in callback TaskGroup._on_done(<Task finishe...> result=None>)
handle: <Handle TaskGroup._on_done(<Task finishe...> result=None>)>
Traceback (most recent call last):
  File "/usr/lib/python3.8/asyncio/events.py", line 81, in _run
    self._context.run(self._callback, *self._args)
  File "/home/user/wspace/aiorpcX/aiorpcx/curio.py", line 147, in _on_done
    self.tasks.remove(task)
KeyError: <Task finished name='Task-29' coro=<PeerManager._refresh_blacklist() done, defined at /home/user/wspace/electrumx/electrumx/server/peers.py:144> result=None>
Exception in callback TaskGroup._on_done(<Task finishe...> result=None>)
handle: <Handle TaskGroup._on_done(<Task finishe...> result=None>)>
Traceback (most recent call last):
  File "/usr/lib/python3.8/asyncio/events.py", line 81, in _run
    self._context.run(self._callback, *self._args)
  File "/home/user/wspace/aiorpcX/aiorpcx/curio.py", line 147, in _on_done
    self.tasks.remove(task)
KeyError: <Task finished name='Task-31' coro=<PeerManager._import_peers() done, defined at /home/user/wspace/electrumx/electrumx/server/peers.py:135> result=None>
INFO:PeerManager:detected SOCKS5 proxy at localhost:9050, auth: none
Exception in callback TaskGroup._on_done(<Task finishe...> result=None>)
handle: <Handle TaskGroup._on_done(<Task finishe...> result=None>)>
Traceback (most recent call last):
  File "/usr/lib/python3.8/asyncio/events.py", line 81, in _run
    self._context.run(self._callback, *self._args)
  File "/home/user/wspace/aiorpcX/aiorpcx/curio.py", line 147, in _on_done
    self.tasks.remove(task)
KeyError: <Task finished name='Task-30' coro=<PeerManager._detect_proxy() done, defined at /home/user/wspace/electrumx/electrumx/server/peers.py:182> result=None>
```

Author: SomberNight

electrumx/lib/util.py

@@ -385,9 +385,12 @@ async def join(self):
 # This is to plug a memory-leak, see https://github.com/kyuupichan/aiorpcX/issues/46 .
 # Note: this breaks the TaskGroup.results and TaskGroup.exceptions APIs
 #       but we are not using them anyway.
+# TODO: this monkey-patch can be removed once we require aiorpcx versions that
+#       have the upstream fix for #46.
 def _patched_TaskGroup_add_task(self: 'aiorpcx.TaskGroup', task):
     self._orig_add_task(self, task)
-    self.tasks.clear()
+    if not hasattr(self, "_retain"):
+        self.tasks.clear()
 
 aiorpcx.TaskGroup._orig_add_task = staticmethod(aiorpcx.TaskGroup._add_task)
 aiorpcx.TaskGroup._add_task      = _patched_TaskGroup_add_task