The grpc::MockClientStream class registers tags on the completion queue, and then fulfills the promises directly rather than going through the typical _processCompletionQueueNotification path. This means that after GRPCReactor::drain() is called, the MockClientStream will leave some tasks "incomplete" (still on the _cqTaskStash in the reactor). This is alright in the MockClientStream, but should be an invariant failure for the non-mock uses of the reactor. Modify the MockClientStream so that we can restore this invariant in the GRPCReactor.