PaddedRSTransport: fix busy loop in poll_sbuffer (follow-up 43ca469774)

According to the asyncio documentation:

> When a task is cancelled, asyncio.CancelledError will be raised in the task at the next opportunity.

I guess the 'next opportunity' means the next await statement.
Here the issue is that the task was not awaiting ever.

Note: ElectrumX needs a similar patch

electrum/interface.py

@@ -359,7 +359,7 @@ class PaddedRSTransport(RSTransport):
             self._sbuffer_has_data_evt.clear()
 
     async def _poll_sbuffer(self):
-        while True:
+        while not self.is_closing():
             await self._sbuffer_has_data_evt.wait()  # to avoid busy-waiting
             self._maybe_consume_sbuffer()
             # If there is still data in the buffer, sleep until it would time out.