IPv6 not working properly on macOS

It turns out that the use of IPv6 with the standard Java [Socket](https://docs.oracle.com/javase/7/docs/api/java/net/Socket.html)/[MulticastSocket](https://docs.oracle.com/javase/7/docs/api/java/net/MulticastSocket.html) classes is [broken when not bound to a specific network interface](https://medium.com/@quelgar/java-sockets-broken-for-ipv6-on-mac-5aae72f06b21), which is breathtakingly lame. The problem doesn't seem to affect Linux or Android.

While I would love for this to be #NotOurProblem, this bug has been present in Java for so long that it doesn't look like it is ever going to get fixed.

This means that if we want we want to support IPv6 when used on a Mac, we will eventually need to take matters into our own hands and explicitly bind to specific network interfaces. Unfortunately, most IoT stuff is going to be using IPv6 exclusively, so we ignore it at our peril.

This issue is for discussing possible strategies for best working around this problem and tracking the eventual fix.