rclcpp::Time and rclcpp::Duration max values not near max int64_t

Follow up Issue to track the conversation in https://github.com/ros2/rclcpp/pull/1347#discussion_r496920298

Basically this is to consider a new approach to the one agreed on #538. The max method for Time and Duration IMO should return the max value of the representation in nanoseconds.

Not sure if that was a constrain in the past, but the sum / substraction operators can handle properly overflow and underflow operations, so I don't see any reason to limit the max value returned by the max method.

If this is not a desired approach, an update to the header of the function will be required, as it stated \return the maximum representable value which is not the case.

Tagging @clalancette for awareness

Start with the rclcpp::Time and rclcpp::Duration max methods and read the conversation linked from pull request #1347, discussion 496920298, together with the earlier decision in issue #538. Determine whether max should return the representation's maximum nanosecond value; done means either that behavior is implemented or the function header accurately documents its intended limit.