Server

[tobyck]

List of changes:

• Send alert threshold in the initial data packet
• Fix bug with height values being sent
• Move a lot of code out of mqtt::start_listening into mqtt:handle_mqtt_message (for tidiness and ease of benchmarking later)
• Make a dev shell for the relay with some extra things for profiling and benchmarking (won't have time before demo but will do for NCEA)

I wouldn't merge this yet, just in case there are other little things that pop up.