AthMessaging: fix for lazy MessageSvc initialization

The setLevel() method triggered the MessageSvc retrieval and if not available prints a WARNING message. This is because we were re-using the message level NIL as an initialization flag. Add an explicit flag for this use-case so setLevel() becomes a trivial method again.

Also re-order member variables to avoid padding.

Thanks to @tadej for reporting this issue in DAOD jobs.