Added support for executors

This pull request replaces the current Node.h with a larger interface which includes executors, groups, timers, and some other common utilities which are required to implement the `ros1_like` examples in https://github.com/ros2/examples/pull/1.
Yeah, it sort of evolved into the wait role, we can rename/reorganize. I guess it means populate_all_handles no matter what (wait or otherwise).