Possible breaking change in protobuf encoding

[cbrake]

Thinking through Simple IoT Project (view), I have concluded we need to update the protobuf encoding to use a double for point value. The float is left over from long ago when points were still samples and optimized for a 32-bit platform. It would be easiest if we make this a breaking change and all instances were upgraded. Let me know if this is a problem for anyone and we can rethink this some more if necessary.