Remove listeners on close()

[ramya-rao-a]

Description

Brief description of the changes made in the PR. This helps in making better changelog

• Removing listeners on the links and sessions in rhea-promise when close() is called on them
• Updating comments for clarity

Reference to any github issues

• When close() is called on a link that got closed due to external factors, the listeners are not cleared #30

[ramya-rao-a]

@ShivangiReja FYI