server: spacing
libraries/github: conform to event chain, simplifications

- Removed a lot of redundant / unecessary logic.
- github plugin only broadcasts that payloads are valid. The eventual goal is that the DB plugin will listen for payload validations, which then trigger DB lookups, which in turn broadcasts whether the given payload has already been seen by the system or not. Subscribed plugins that actually care about new, old or updated payloads will simply listen for those messages and react accordingly.