till-varoquaux

#pragma once
/**
 * This is a quick abstraction over non-blocking asynchronous deferreds. In a
 * way you can see those as non-blocking futures (whereas std::future are
 * blocking).
 * This implementation is devoid of fanciness. Amongst other things:
 *  - Deferreds are not cancelable
 *  - They are not threadsafe. Callbacks are ran in whichever thread the
 *    deferred was triggered and there's no mutex to protect from race condition
 *    while registering callbacks/setting values. This is what we use libev