Clean up receive_buffer. Fixes memory leak. (#141)

Refs django/channels#1181

channels_redis/core.py

@@ -383,6 +383,7 @@ async def receive(self, channel):
  except asyncio.CancelledError:
  # Ensure all tasks are cancelled if we are cancelled.
  # Also see: https://bugs.python.org/issue23859
+ del self.receive_buffer[channel]
  for task in tasks:
  if not task.cancel():
  assert task.done()
@@ -433,6 +434,9 @@ async def receive(self, channel):
  else:
  self.receive_buffer[message_channel].put_nowait(message)
  message = None
+ except:
+ del self.receive_buffer[channel]
+ raise
  finally:
  self.receive_lock.release()