js: disallow bulk set data on key from outside

• src/Key.js Key class is not exported anymore, as it should not be used directly anywhere. setKeyData is no more a method of the Key, (optional) data are now validated and set on Key creation and on updates, both from within this module, thus no longer exposing setKeyData to the outside.
• createKey now gained an optional parameter which allows to set Key data at this point.