MQTT support

[jonnor]

Alternatives for implementing

• https://github.com/Ather-Energy/RuMqtt pure Rust
• https://github.com/inre/rust-mq, pure Rust client. Separate mqtt3 crate for protocol

Not looking so good

• https://github.com/zonyitoo/mqtt-rs looks like pure Rust, but not much info on maturity
• https://github.com/cubehub/rust-mqtt wraps Paho C library
• https://github.com/tkellogg/rust-mqtt pure Rust, but not updated since 2015