This is a fairly simple addition of a Redis-backed TokenPersistence. When GithubConstellation is initialized, it will create a FsTokenPersistence or a RedisTokenPersistence based on configuration. Have added tests of the Redis backend as an integration test, and ensured the server starts up correctly when a `REDIS_URL` is configured.
Ref: #1848
Instead of saving tokens on a timer, save them when they change. Use EventEmitter to keep the components loosely coupled.
This is easier to reason about, much easier to test, and better supports adapting to backends which support atomic operations.
This replaces json-autosave, which was a bit difficult to read and also hard to test, with fsos, the lower-level utility it’s built on.
Ref: #1848
This creates a new convenience class which consolidates all the Github initialization. It supports dependency injection and facilitates refactoring the persistence along the lines of #1205.
Also ref #1848
