Add concept of release streams to the plugin and API

We need to be able to test new features (and potentially support custom builds?) in a backwards-compatible manner, so server and client builds are not locked to one another. In other words, we need a versioning system flexible enough to support branching versions, and that can negotiate supported features between clients and servers.

**Notes**
- Add version check endpoint
- Needs to handle both servers and clients being outdated
- Protocol version?
- Needs to ideally be compatible across release streams for convenience in development
- Enables using a stable build to connect to a nightly server and vice versa
- Feature declaration endpoint?

Start by mapping the plugin and API entry points involved in client-server communication; the issue names no files or tests. Define how release streams, protocol versions, outdated peers, and feature declarations should be negotiated, including stable/nightly compatibility. Done means the version-check and feature-negotiation behavior is specified and implemented for both clients and servers.

Written by the indexing model from the issue text.