Rate limiters for API scalability

From nexus-kg created by [umbreak](https://github.com/umbreak) : BlueBrain/nexus-kg#31

We need to have a way to ensure the quality of our service in cases of unexpected increase in traffic or overuse by some customer.

[Scaling your API with rate limiters](https://stripe.com/blog/rate-limiters) explains the different types of limiters you can define on an API to prevent from those traffic bursts.

That article defines 4 rate limiters. Some are easy to implement and some might be more difficult. We should start by:

- Request rate limiter: Restricts each user to N requests per second.
- Concurrent requests limiter: Restricts each user to N simultaneous requests.

I would argue that this is not high priority yet, but at some point it has to be tackled.