keys: store type and rev in hash rather than serialized

Since the old keys used to be strings i started out by
json serializing the new keys with type, value, rev.

However storing serialized json in couch (json) does
not really make sense. So now we do not serialize but
instead have one json document. The lookup for a key of
type pgp may still return a string but for everything
that uses the new api it will return a hash with type
and revision.

This data structure is way easier to handle also on the
nickserver side.