reuse logic from Time::durationToSec method

[dirk-thomas]

As suggested in https://github.com/ros2/rclcpp/pull/536#discussion_r211035479 the logic could be moved to rcutils and be used in rclcpp::Time and rclcpp::Duration.

[CursedRock17]

I can open up a PR for this since it's a simple logic movement but I would assume it would make more sense to move the logic to rclcpp opposed to rcutils to keep rcutils concise. Does anybody else have any opinions on where it should be moved to?

[clalancette]

I can open up a PR for this since it's a simple logic movement but I would assume it would make more sense to move the logic to rclcpp opposed to rcutils to keep rcutils concise.

I don't think we can do that. The problem is that tf2 does not, and cannot, depend on rclcpp. However, both tf2 and rclcpp currently depend on rcutils, so that is a logical place to share the implementation.