Add entry-cleaning up behavior when stream got dropped

[solus161]

Description

What's new:

• Add a guard for SubEntry for RAII behavior. Also update stream in ClientManager.subscribe() to take in the guard and the Receiver;
• Add test for the entry-cleaned-up behavior when stream is dropped;

Related Issue

• Closes [BUG] Dropped WS subscriptions remain registered until a matching broadcast #206

Checklist

• I have read the CONTRIBUTING.md document.
• My code follows the project's coding standards.
• I have added tests to cover my changes.
• All new and existing tests passed (make check).
• I have updated the documentation accordingly.