Make __init__ consitstent across Store heirarchy

Add db_conn parameters to the `__init__` methods of the *Store classes, so that
they are all consistent, which makes the multiple inheritance work correctly
(and so that we can later extract mixins which can be used in the slavedstores)

synapse/storage/_base.py

@@ -162,7 +162,7 @@ class PerformanceCounters(object):
 class SQLBaseStore(object):
     _TXN_ID = 0
 
-    def __init__(self, hs):
+    def __init__(self, db_conn, hs):
         self.hs = hs
         self._clock = hs.get_clock()
         self._db_pool = hs.get_db_pool()