SpineEventEngine
Substitute `MessageEnvelope`s with interfaces with default and static methods
Currently, many messages in the framework have associated Envelope types, e.g. CommandEnvelope, EventEnvelope. This is semantic duplication. We should use interfaces with (is) option instead.
It's a nice issue, and I would like to get rid of envelopes some day. But EventMessage != Event.
We can probably, re-name Event to something like EventObject, and have the Event interface, which EventObject message implements. Then Event could probably have all the things that now EventEnvelope exposes.
@armiol, what's your view on this matter. It's a significant change, which is nice, but should be done after v1.0.0 is out and other important matters are addressed. It's a server-side internal thing. We can polish it when time permits.