ZODBClientThread.get_thread_dict(): check7ZODBThreads fails often on

Fred's machine, due to this routine falling off the end and returning
None.  Sync'ing the connection after a ConflictError appears to fix
it, or at least make it much less likely to fail.

src/ZODB/tests/MTStorage.py

@@ -96,6 +96,7 @@ class ZODBClientThread(TestThread):
                 return root.get(name)
             except ConflictError:
                 get_transaction().abort()
+                root._p_jar.sync()
 
 class StorageClientThread(TestThread):