Add database config class (#6513)

This encapsulates config for a given database and is the way to get new
connections.

synapse/handlers/presence.py

@@ -230,7 +230,7 @@ class PresenceHandler(object):
         is some spurious presence changes that will self-correct.
         """
         # If the DB pool has already terminated, don't try updating
-        if not self.hs.get_db_pool().running:
+        if not self.store.database.is_running():
             return
 
         logger.info(