connects to ros2/rcutils#57

currently including commits from #130; if this PR is ready before ros2/rcutils#57 gets merged then it can replace #130.

This PR exposes the new notions from ros2/rcutils#57:

1. there's a default severity threshold,
2. each logger can have its own severity threshold specified,
3. there's a hierarchy of loggers: I've added a get_child() method on loggers; children inherit the severity threshold of their parent if set (the severity threshold inheritance taken care of by rcutils now that it has a notion of logger hierarchy)
4. all loggers at some point are attached to the root rclpy logger (which is where they get their default severity threshold)

Out of scope:

1. node.get_logger() or any notion of attaching loggers to nodes (waiting on decision in Prototype use of rclpy logging for callback_group examples#186)
2. any caching of whether or not loggers are enabled and updating based on notifications when severity levels change