Merge "Don't reject "ctl.stop.*" during shutdown." into main

                                   SocketConnection* socket, std::string* error) {

    auto lock = std::lock_guard{accept_messages_lock};

    if (!accept_messages) {

        // If we're already shutting down and you're asking us to stop something,

        // just say we did (https://issuetracker.google.com/336223505).

        if (msg == "stop") return PROP_SUCCESS;

        *error = "Received control message after shutdown, ignoring";

        return PROP_ERROR_HANDLE_CONTROL_MESSAGE;

    }