[UNSTABLE] RemoteEndpointRegistry, AckedDelivery

Working on implementing the `EndpointActor` built-in actors.

This pull request includes:

* Completed `EndpointRegistry` and specs
* Completed `EndpointManager` - but still working on some details
* `AckedDelivery` collections and sequencing, plus initial specs.

I decided that forgoing AckedDelivery in this build actually created more problems (in terms of expedient delivery of V1) than it solved, so I'm working on integrating it into remoting as part of issue #1