Consolidate reactor timer management functions

[sonndinh]

• Problem: ACE_Reactor and its base class, ACE_Reactor_Timer_Interface have identical implementations for schedule_timer and reset_timer_interval member template functions.

• Solution: Bring the base class's member functions into the scope of the ACE_Reactor class with using.

The original attempt was to remove the duplicate member functions from ACE_Reactor while keeping the ones from the base class. However, this does not work since the base class's member template functions are not visible in ACE_Reactor with the presence of the overloads for schedule_timer and reset_timer_interval with ACE_Time_Value. This change brings the base class's schedule_timer and reset_timer_interval member templates into the scope of the ACE_Reactor class so that we no longer need the identical implementations in ACE_Reactor.

[sonndinh]

@mitza-oci Using using is better - it's more concise and saves a call to the base class's member function.