Fixed the "potential ZODB cache inconsistency after client reconnect"

problem reported on zodb-dev:

http://mail.zope.org/pipermail/zodb-dev/2006-August/010343.html

Added a new invalidateCache protocol for DBs and Connections to
invalidate the entire in-memory caches.  This is used when ZEO clients
reconnect.

src/ZEO/tests/ConnectionTests.py

@@ -79,6 +79,9 @@ class DummyDB:
     def invalidate(self, *args, **kwargs):
         pass
 
+    def invalidateCache(self):
+        pass
+
 class CommonSetupTearDown(StorageTestBase):
     """Common boilerplate"""