Restore call to setLastTid() in tpc_finish().

It was lost backporting changes from the 3.1 branch, which doesn't
have the last tid feature.

src/ZEO/ClientStorage.py

@@ -900,7 +900,8 @@ class ClientStorage:
             finally:
                 self._lock.release()
 
-            self._server.tpc_finish(self._serial)
+            tid = self._server.tpc_finish(self._serial)
+            self._cache.setLastTid(tid)
 
             r = self._check_serials()
             assert r is None or len(r) == 0, "unhandled serialnos: %s" % r