Refactor the HS code to support both current and proposal 224 design
This is an important task since we need both system to live in parallel for multiple years and before we start the heart of the work on proposal 224, we need to accomodate the code so we avoid huge amount of code duplication.
Would be also a good opportunity to add unit tests while doing so!
Here are some notes on a possible start up point to refactor: https://people.torproject.org/~dgoulet/hs224-refactor.txt