Method notify

test/bthread_cond_unittest.cpp:264–268

protected:
    Signal() : _signal(0) {}
    void notify() {
        BAIDU_SCOPED_LOCK(_m);
        ++_signal;
        _c.notify_one();
    }

    int wait(int old_signal) {
        std::unique_lock<bthread::Mutex> lck(_m);