Add chrono APIs to DeadlineTimer

modules/beast_core/thread/DeadlineTimer.h

@@ -63,8 +63,17 @@ public:
                                     will send a notification. This must be
                                     greater than zero.
     */
+    /** @{ */
     void setExpiration (double secondsUntilDeadline);
 
+    template <class Rep, class Period>
+    void setExpiration (std::chrono::duration <Rep, Period> const& amount)
+    {
+        setExpiration (std::chrono::duration_cast <
+            std::chrono::duration <double>> (amount).count ());
+    }
+    /** @} */
+
     /** Set the timer to go off repeatedly with the specified frequency.
         If the timer is already active, this will reset it.
         @note If the timer is already active, the old one might go off