Fix regression in change tracking

Make sure we update the referenced change object on a new gerrit
event rather than waiting to remake the queue item.

This was a performance regression in the connection changes.

Change-Id: I2a967f0347352a7674deb550e34fb94d1d903e89

zuul/connection/__init__.py

@@ -43,6 +43,14 @@ class BaseConnection(object):
         self.connection_name = connection_name
         self.connection_config = connection_config
 
+        # Keep track of the sources, triggers and reporters using this
+        # connection
+        self.attached_to = {
+            'source': [],
+            'trigger': [],
+            'reporter': [],
+        }
+
     def onLoad(self):
         pass
 
@@ -51,3 +59,6 @@ class BaseConnection(object):
 
     def registerScheduler(self, sched):
         self.sched = sched
+
+    def registerUse(self, what, instance):
+        self.attached_to[what].append(instance)