Unit tests and cleanup for outgoing federation code (part 2)

[Nutomic]

[phiresky]

Please either base these changes on #4623 or wait for that to finish. I've made refactorings in there and all of this will double that work because it's basically the same changes in a different way, and it's very demotivating to see work wasted.

[phiresky]

Especially simple refactorings like renaming a variable cause huge amount of work when merging for minimal benefit.

[phiresky]

fyi: I've copied the unit tests part of this to #4559 and fixed it a bit, I'll work more on it the next days.