remove a race when checking whether we need to update the cache tid after verification.

src/ZEO/ClientStorage.py

@@ -1390,6 +1390,7 @@ class ClientStorage(object):
             server.verify(oid, tid)
         server.endZeoVerify()
 
+        with self._lock:
             if server_tid > self._cache.getLastTid():
                 # We verified the cache, and got no new invalidations
                 # while doing so.  The records in the cache are valid,