Feature/unsubscribe from multiple topics

[SivaAkiro]

Motivation:

Multiple MQTT topics can be unsubscribed together.

Conformance:

Your commits should be signed and you should have signed the Eclipse Contributor Agreement as explained in https://github.com/eclipse/vert.x/blob/master/CONTRIBUTING.md Please also make sure you adhere to the code style guidelines: https://github.com/vert-x3/wiki/wiki/Vert.x-code-style-guidelines

[SivaAkiro]

Fixed the review comments, can you please review?

[SivaAkiro]

Nothing is changed, wrongly clicked on the refresh button.