Rework `MessageId`

[alexander-yevsyukov]

As the analysis of this code related to this issue (#1189) reveals, the MessageId type and the code around it is puzzling. The name of the type conflicts with what we have already in the framework language. We have already CommandId, and EventId. Also, we promote typed entity IDs. And when the public API shows also MessageId, it raises a question: “What the hell is this? Why do you need it, and why do I need to know about it?”

More, the “guts” of the type are more than ID: we have type URL of the identified message, as well as the version. Sometimes, MessageId produces a copy of itself with a newer version. We definitely need a better name.

Also, exposure of MessageId to the client API (via EventContextMixin) is unnecessary. Presumably, the MessageId types serves for diagnostics, and as such should be isolated to the servers-side code.