asyncioreactor: initial background_tasks set earlier

in b80960f we introduce this new set, but initialize it
after starting the coroutines, which can lead to cases
it won't yet be defined.

moveing it to the start of the the `__init__` method fixes
the issue

Author: fruch

cassandra/io/asyncioreactor.py

@@ -87,6 +87,7 @@ class AsyncioConnection(Connection):
 
     def __init__(self, *args, **kwargs):
         Connection.__init__(self, *args, **kwargs)
+        self._background_tasks = set()
 
         self._connect_socket()
         self._socket.setblocking(0)
@@ -106,7 +107,7 @@ def __init__(self, *args, **kwargs):
         )
         self._send_options_message()
 
-        self._background_tasks = set()
+
 
     @classmethod
     def initialize_reactor(cls):