Wip/mqtt/init - tracking mqtt experiments

The corresponding uploader changes (there are a few actually)

The protobuf model:
https://github.com/ktind/android-uploader/blob/mqtt-protobuf/app/src/main/java/com/nightscout/android/protobuf/SGV.proto


Feel free to close/ignore if this gets in the way of things, I'll move it to my personal fork if it becomes a distraction.