Store subscription indentifier into subscriptions trie

[andsel]

Release notes

Store and retrieve the subscription identifier into the subscriptions directory.

What does this PR do?

Spread the SubscriptionIdentifier of a subscription request down to the SubscriptionDirectory.
Updated the CNode 's addSubscriptionto update subscriptions stored also if the subscription identifier changes (by value, removing or adding int).

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

It's a step in the implementation of subscription id functionality

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)
• 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 Implement Subscription identifier #801

Use cases