Hedged HTTP client which helps to reduce tail latency at scale (library)

 See paper Tail at Scale (https://cacm.acm.org/magazines/2013/2/160173-the-
 tail-at-scale/fulltext) by Jeffrey Dean, Luiz André Barroso. In short: the
 client first sends one request, but then sends an additional request
 after a timeout if the previous hasn't returned an answer in the
 expected time. The client cancels remaining requests once the first
 result is received.
 .
 Features
 .
  * Simple API.
  * Easy to integrate.
  * Optimized for speed.
  * Clean and tested code.
  * Supports http.Client and http.RoundTripper.
  * Dependency-free.