Core redesign

[h2non]

Currently a big portion of internal design is poor, ugly and not very well designed.
The code hard to maintain and hard to extend.

This kind of lack is since resilient.js was initially designed as experimental project underestimating a considerable complexibility growth. Most of the design efforts were placed into the external API, trying to simplify it for the best usage, becoming really easy to use and underlining complex technical details to library consumers, instead of a investing more efforts designing it properly at internal level.

Next version should be completely redesigned with a tiny core, completely based on plugins and middleware, mainly inspired by the Go port