Unsubscribe shared subscriptions

[andsel]

Release notes

Implemented the unsubscribe for shared subscriptions.

What does this PR do?

Updates the CTrie.removeSubscriptionsFor to accept an UnsubscribeRequest instead of simple parameters, customizing the method to handle a shared subscription's unsubscribe request.
Adds a method removeSharedSubscriptionsForClient to remove in one step all the shared subscriptions of a session, in doing so it introduces a mapping cache clientId -> list of subscription to avoid a full tree traversal.

Why is it important/What is the impact to the user?

Implements part of MQTT5 shared subscription feature

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files (and/or docker env variables)
• [ x I have added tests that prove my fix is effective or that my feature works
• I have updated the Changelog if it's a feature or a fix that has to be reported

Related issues

• Relates [Meta] Implement Shared Subscriptions #791