refactor: custom `NetworkError` and move code into relevant modules

[de-sh]

Changes

• Create custom NetworkError type
• Move readb from Network into MqttState
• Move timeouts into Network::flush()
• Reorganize imports

[tekjar]

It's a good idea to pass Network to MqttState instead of passing MqttState to network. This allows us to easily separate networking implementation from MQTT state management. I'll discuss these changes with @yatinmaan and merge this.