use _close_old_connections in channels.signals

XXX: Django still uses its broken method for
request_started/request_finished though.

Author: blueyed

channels/signals.py

@@ -1,8 +1,8 @@
-from django.db import close_old_connections
+from channels.db import _close_old_connections
 from django.dispatch import Signal
 
 consumer_started = Signal(providing_args=["environ"])
 consumer_finished = Signal()
 
 # Connect connection closer to consumer finished as well
-consumer_finished.connect(close_old_connections)
+consumer_finished.connect(_close_old_connections)