MozPromise: A Better Tool for Asynchronous C++

I suggest use libprocess in https://github.com/apache/mesos/tree/master/3rdparty/libproc... Is it more widely used and test well.

Having used JavaScript and C++ promises, the thing I find easiest is coroutines, and they've been accepted for C++17. In the meantime Boost Coroutine together with ASIO makes for very readable code.