Constructor to set a particular expiry time as an absolute time.
basic_waitable_timer( boost::asio::io_service & io_service, const time_point & expiry_time);
This constructor creates a timer and sets the expiry time.
io_service object that
the timer will use to dispatch handlers for any asynchronous operations
performed on the timer.
The expiry time to be used for the timer, expressed as an absolute time.