Support RateLimit draft

[ioggstream]

I wish

Sphinx to support this RateLimit Draft proposal (eg. via a configuration parameter):

• https://www.ietf.org/id/draft-polli-ratelimit-headers-03.html

The proposal has been already implemented by Kong and RedHat 3Scale API Gateway.

Differences respect to the current implementation

The current implementation returns X-RateLimit-Reset with a unix timestamp.
This approach is not aligned with the Retry-After http header and is subject to clock skew from clients.

To fix it, we should just change the following line - eg. via a configuration parameter -

sphinx/handlers/http.go

Line 107 in 2a1eb2c

headers["X-Ratelimit-Reset"] = append(headers["X-Ratelimit-Reset"], int64ToString(status.Reset.Unix()))

subtracting the current date.