SPIKE: improve design of authn/z

TBD:
– summarise current design and refer to existing docs
– propose API changes
– propose additional utilities (like caching)

Goal

This goal is to improve the following aspects of the existing authn/z:

• improve performance and cacheability – structure the API to limit the number of token signing requests

• standardize and simplify mod-authtoken API – use regular RAML/JSON API defintion

• lift header-size limites – move away from the header-oriented API, limit the number of custom headers

https://docs.google.com/document/d/14DVZC3MJRQfgNchMXzxtz4-2s0W9pmnXCLq5KbjeKaw/edit?usp=sharing