Rather than accessing the Bridge user list, each user register their
individual SMTP service with the server manager.
Note that some dependencies on the user data are hidden behind the
`UserInterface`. These will be removed in a future patch.
* Remove distinction between values with and without reply.
* Hide types that don't need to be public.
* Don't allow direct access to the request's internal types.
Update ServerManager to follow the new expected behavior. The servers
will only be started when one user is active.
If all users are logged out or removed from the system, the servers will
stop.
If the network goes down, the servers will stop and resume once network
has been restored.
Add a dedicated go-routine whose sole responsibility is to manage the
life time of the IMAP and SMTP servers and their listeners.
The current implementation behaves the same way as the previous state.
The new behavior will be implemented in a follow MR.
