Peter M. Groen / TrueMQTT-cpp

This includes CONNACK and SUBACK.

By using Happy Eyeballs, we stagger connections of a host resolves
into multiple IPs. This is useful for IPv6 / IPv4 hosts, where
one of the two can stutter.

Sadly, creating a connection is rather complex, with many odd
things that can happen along the way. For example, a writeable
socket doesn't mean it is actually connected; it can also mean
the socket is in an error state.

This implementation is inspired by my own work on OpenTTD's
variant of this.

All administration should been taken care of now, and the only
thing remaining is creating connect/disconnect and implementing
the sendXXX functions.

This contains no actual code yet, just the scaffolding to get
started.