Rework inner concepts of UStreamer to incorporate better encapsulation and domain concepts

@sophokles73 was kind enough to sit with me and outline some changes he think would improve maintainability.

They can be summarized as: improve encapsulation and use of domain concepts.

For example, we currently have the concept of an `Endpoint`. When we connect two ends together, we're effectively creating a _route_, so he suggested seeing if we can't incorporate a `Route` type to better hide some details and avoid having multiple thread-safe hash maps.