Unique Worker ID

[whilenot-dev]

Summary

Following this discussion, this is a draft to inject a unique worker ID into the app state of an application. Feedback welcome!

The worker ID should be consistent across restarts and provide the missing building block for idempotent startup- and shutdown routines in the lifespan of a worker.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[whilenot-dev]

Before I'll tackle the test coverage I'd like to have some feedback first:

1. Should I limit the worker ID injection to multiprocess-mode only?
2. Should I document the injection more like a warning? Can the injection be considered a bad sursprise?
3. How about the simplicity, @Kludex? :D