Synchro: Update class barrier with a completion function

[viboes]

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

add a completion function associated to the class barrier

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

        barrier(unsigned int count);
        template <typename F>
        barrier(unsigned int count, F&&);

        ~barrier();

        bool wait();
        void count_down_and_wait();
    };