MailboxListener may not removed even if MailboxListener.isClosed() is true

At the moment we check on every new even for a specific mailbox if an listener was closed before and if so we remove it. This is error-phrone and also may lead to "leaking" MailboxListeners as if no new event for a Mailbox will submitted it will there forever.

We should better directly remove the MailboxListener and not relay on a lazy removal. This also should be a way faster as we don't need to check all MailboxListeners for a Mailbox on every event