readd some privacy

[tharvik]

readding what was called differential privacy which is quite a bit harder to implement correctly (we need to know the variability of every features for one, and have a dedicated paper-based way to implement it, see #613).
I got lost along the way and changed more things, oupsi

• reexpose privacy module, with addNoise and clipNorm doing what was doing goo'old DP
  • with tests™©®
  • enable it in mnist task via TrainingInformation.privacy
• flatten DistributedTrainer, LocalTrainer, TrainerBuilder into a simple non-abstract Trainer
• change discojs.Disco to a more straightfoward object
  • exposing only the contained Trainer and not Logger, Task and others
  • have a clearer code on where the model is taken from
  • ensure that it gets passed some valid client options
• upgrade Aggregator to a real event emitter
  • avoid weird promises use (makeResult, receiveResult 👀)
  • drop Model link, everything goes through events now
  • drop mocking in server's tests
  • drop unused AsyncInformant

[martinjaggi]

amazing PR, thanks so much. looks so much cleaner now and will be much easier to extend in the future. well done!

[JulienVig]

Great work! The codebase is getting cleaner and cleaner