We do not want to send a `session` related to a single node context, other a message queue.
Modify the Event API and implementation to just be exposing a `User`.
Rework MailboxListeners so that they either:
- Directly reuse the User where possible
- Reopen system sessions for that given user if needed.
You have to replace getSession() by a new getUser() in Event class. In some listeners, they requires to retrieve Session, let's use MailboxManager to create mailbox session of the user from getUser()