keymanager - check if key it's a newer version of a known one

On a upgrade of the same key check if it's a newer version of the same key or an attacker giving us the old version.

Maybe checking the date of the signatures of the new parts (http://tools.ietf.org/html/rfc4880)

(from redmine: created on 2014-10-12, closed on 2014-11-10)