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Constructor to set a particular expiry time relative to now.
basic_waitable_timer( asio::io_service & io_service, const duration & expiry_time);
This constructor creates a timer and sets the expiry time.
                  The 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, relative to now.