Substitute `MessageEnvelope`s with interfaces with default and static methods

[dmdashenkov]

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.

[alexander-yevsyukov]

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.

[alexander-yevsyukov]

@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.