Revert "Use weakref for change cache"

This reverts commit b9704302bd.

This is strongly suspected of causing a memory leak.

Change-Id: I0ebf9cee304277909a0b80420ac7ba659a437b29

zuul/connection/__init__.py

@@ -68,6 +68,13 @@ class BaseConnection(object, metaclass=abc.ABCMeta):
     def registerScheduler(self, sched):
         self.sched = sched
 
+    def maintainCache(self, relevant):
+        """Make cache contain relevant changes.
+
+        This lets the user supply a list of change objects that are
+        still in use.  Anything in our cache that isn't in the supplied
+        list should be safe to remove from the cache."""
+
     def registerWebapp(self, webapp):
         self.webapp = webapp