Leverage rcl functions to get topic of publisher/subscription

[minggangw]

This patch implements:

1. Return the result of rcl_publisher_get_topic_name() directly, instead of splitting it by "/" and returns the last part.
2. Use rcl_subscription_get_topic_name() to get the topic of a subscription.

Meanwhile, this patch updates the following tests:

• test/test-node-oo.js
• test/test-node.js
• test/test-remapping.js

Fix: #949, #950