Light client roadmap: REST API rate limiter

[zsfelfoldi]

Goal: make the API service safe for public service
Status: proposal/experiment, a PoC implementation and a detailed description of the proposed scheme is WIP

Note: unlike the old LES flow control model, this scheme does not require assumptions about request costs in advance, it completely avoids cost tables and all the complexity that came with it, and will hopefully still achieve a better perfomance in normal cases (still needs to be proven in practice though). Also, it can be specified as an entirely optional performance optimization.

Relevant branch:

• https://github.com/zsfelfoldi/go-ethereum/tree/blsync-ratelimit