Revamp Time

[clalancette]

This PR does the bare minimum to implement 'use_sim_time' in rcljava. By 'bare minimum', I mean:

• It revamps the Time class to be more like the rclcpp/rclpy equivalents.
• It revamps the Clock class to be more like the rclcpp/rclpy equivalents.
• It implements the TimeSource class, which is responsible for setting up the 'use_sim_time' parameter and subscribing to the '/clock' topic.
• The current tests pass.
• New tests for the revamped Time class were added.
• No new tests for Clock or TimeSource were added.
• There are still a few (relatively minor) TODO comments in the code.
• I didn't test that this actually, you know, works.

Nevertheless, I think this is a good starting point to finish implementing 'use_sim_time'. The next steps here are to write tests, fix the TODOs, and then test that 'use_sim_time' actually works.

This replaces #5 .

@jacobperron @ivanpauno FYI.

[jacobperron]

I've merged this into a feature branch. I'll continue iterating there, adding documentation and tests.