asyncioreactor: make sure task isn't deleted midway

in push function, self._loop.create_task is called and it's return value is ignored.
While the tests may pass now, this code is not correct and this example is called out
in docs as a source of bugs, as python docs suggests.

Ref: https://docs.python.org/3/library/asyncio-task.html#asyncio.create_task

cassandra/io/asyncioreactor.py

@@ -106,6 +106,8 @@ def __init__(self, *args, **kwargs):
         )
         self._send_options_message()
 
+        self._background_tasks = set()
+
     @classmethod
     def initialize_reactor(cls):
         with cls._lock:
@@ -176,7 +178,10 @@ def push(self, data):
             )
         else:
             # avoid races/hangs by just scheduling this, not using threadsafe
-            self._loop.create_task(self._push_msg(chunks))
+            task = self._loop.create_task(self._push_msg(chunks))
+
+            self._background_tasks.add(task)
+            task.add_done_callback(self._background_tasks.discard)
 
     async def _push_msg(self, chunks):
         # This lock ensures all chunks of a message are sequential in the Queue