Sync: Add a latch class

[viboes]

Based on N3600- C++ Latches and Barriers ​http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3600.html

implement a latch class that could have the following interface

    class latch
    {
    public:
        latch(latch const&) = delete;
        latch& operator=(latch const&) = delete;

        latch(std::size_t count);
        ~latch();

        void wait();
        bool try_wait();
        template <class Rep, class Period>
        cv_status wait_for(const chrono::duration<Rep, Period>& rel_time);
        template <class lock_type, class Clock, class Duration>
        cv_status wait_until(const chrono::time_point<Clock, Duration>& abs_time);
        void count_down();
        void count_down_and_wait();
        void reset(std::size_t count);
    };

[viboes]

​https://github.com/boostorg/thread/commit/750c849b0f0dff79a289111955260a4147ac7f59

[anonymous]

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