Apply snappy compression to all message bus traffic

[kevsmith]

Use Elixir and Go snappy implementations to transparently compress and decompress all message bus traffic.

Cog

• lib/carrier/messaging/connection.ex is where we should add compression since all messages are published by calls into this code.
• Implementing decompression is a bit trickier as Cog components rely on receiving MQTT messages as Erlang messages. As a start we can fix up the few places where we're matching on {:publish, ...} to handle compressed payloads.

Relay

• We can handle transparent compression and decompression in relay/bus/mqtt.go.