removeAllSessions() to clean up connection.

[ShivangiReja]

This PR adds a new function removeAllSessions() that calls remove_all_sessions() method of rhea that calls on the connection to clear the internal map for sessions.

Reference : amqp/rhea#207

cc @AlexGhiondea @ramya-rao-a

[ramya-rao-a]

@amarzavery FYI

[amarzavery]

The PR LGTM.

A side comment: The Connection class has a method to createSession(). rhea-promise does not maintain an internal map of rhea-promise session objects created on the rhea-promise connection object. The servicebus or the eventhub sdk or the amqp-common layer that uses rhea-promise usually maintain those session objects. So make sure those sessions are also cleared in the above mentioned libraries whenever they are created.

[ramya-rao-a]

Updating the comments on all the remove and closes in this library in #34 along with removing the listeners when close() is called