Add getClock to Node object

[mattrichard]

Adds an internal clock to rclnodejs nodes. In addition this also makes the following changes:

• Timer handles will now be children of the clock handle. This matches rclpy's implementation.
• Parameter events will use the node's clock instead of creating a new one each time.
• Exposes ClockType publicly.

Actions will also require use of the node's clock.

Fix #None

[minggangw]

Thanks for implementing this function LGTM