keymanager - upgrade key when signed by old key

(3) key replacement: A registered key MUST be replaced by a new key in one of the following situations, and ONLY these situations:
    (a) verified key transitions (when the new key is signed by the previously registered key for same address).

(from redmine: created on 2014-10-21, closed on 2015-01-15, relates #6120 (closed))