fix(voip-server-mumble): cleanup channel listeners when client is freed

[kajkul]

Goal of this PR

Remove mubmle channel listeners when a client gets freed in order to prevent newly connected users form listening to channels that were left behind by the previous owner of that session id.

How is this PR achieving the goal

It adds new methods to the ChannelListener class that erase all the channels the user is listening to from both maps at once. I didn't use the exsiting API since that would mean holding the mutex per channel instead of once for the whole operation.

This PR applies to the following area(s)

Server

Successfully tested on

Platforms: Windows

Checklist

• Code compiles and has been tested successfully.
• Code explains itself well and/or is documented.
• My commit message explains what the changes do and what they are for.
• No extra compilation warnings are added by these changes.