Fix timers

[mauropasse]

Needs: irobot-ros/rcl#25

[mauropasse]

    static bool timer_greater(TimerPtr a, TimerPtr b)
    {
      return a->next_call_time() >= b->next_call_time();
    }

I don't remember why we started using next_call_time instead of the previous time_until_trigger.
Was there a reason for next_call_time?
Something related to this?
// We don't use rclcpp::Time::max() because we want to specify the clock type

[mauropasse]

Closed since not all changes necessary