fix: subscribing to a message channel while unsubscribing is pending [MTT-3756]

[LPLafontaineB]

Description

This PR makes sure that when subscribing to a message channel after having previously subscribed and unsubscribed to it works properly, even if no messages have been published in the mean time. To do so, we keep the list of pending additions and removals from the list of handlers in a dictionary. If we subscribe while a pending unsubscribe is in the dictionary, we simply remove it from the dictionary instead of adding a new pending subscription, and we do the same when unsubscribing if we have a pending subscription.

Issue Number(s)

MTT-3756

Contribution checklist

• Tests have been added for boss room and/or utilities pack
• Release notes have been added to the project changelog file and/or package changelog file
• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• JIRA ticket ID is in the PR title or at least one commit message
• Include the ticket ID number within the body message of the PR to create a hyperlink

[pdeschain]

We discussed it in Slack, implementation looks good 👍

[SamuelBellomo]

Let's add tests for this case, so there's no regression in the future. This way, even though we don't have a use case in game, we'd have tests to make sure this behaviour doesn't change later.