Create an equivalent to ros::Time

[tfoote]

This should provide a standard way to communicate simulated time and present it to the developer through the client libraries in as native a manner as possible.

• create a design document Add a document for ros_time design#72
• core implementation in rcl first work toward adding ros time support rcl#12
• implement a native cpp interface in rclcpp
  • rclcpp::Time::now(): Rclcpp time rclcpp#311
• implement a native c interface in rcl
• implement a native python interface in rclpy Implement Time Features in rclpy rclpy#186
• extend use of rclcpp:Time features to existing Rate and Timer Extend Time implementation into Timer and Rate objects rclcpp#465

[tfoote]

Design document: ros2/design#72

[mikaelarguedas]

will be picked up in the near future for partial implementation needed for several already in progress tasks (links to be added)