Add session expiry for database cleanup

Nothing is done with this expiry at the moment, it is only extended if
the session would expire within one day. If a session was already expired,
nothing happens, it will just be extended (SubminSessionID is re-used).

Old sessions will be purged later.
Added session cleanup, removed superfluous save()

Sessions are now cleaned up when someone logs in. Not all sessions are
cleaned up, only the sessions that expired and up to a limit of 100 to
prevent long delays.