Remove the requirement of double-encoded json messages

[supertick]

Describe the bug
parameters currently are double encoded within a json message definition
Remove and fix external clients ...

[AutonomicPerfectionist]

I think double encoding is fine, personally. Sometimes we do need the information from the message itself in order to deserialize the data parameters, even with the embedded class field. An example is given in #1289 actually, it uses the destination name of the message to lookup the expected data type to deserialize into, which strengthens type safety and wouldn't be easy to do without double encoding