JS Mqtt - PR #2 Data model and packet logging

[bretambrose]

Additional packet definitions and helper functions for the internal JS MQTT client. Beyond what we currently expose in mqtt5_packet, we add two additional models:

1. An internal model that extends the public model of mqtt5_packet with fields we don't want to publicly expose (packet id, clean start, etc...), We decode into this model.
2. A binary model (bytes or numbers only) for all operations, internal and external. We do this to ensure we only pay the binary conversion cost of strings to bytes once. This is the model we encode and write to the socket.

Also includes logging related functionality:

1. Additional helper functions for function-based logging that require a log level check pass before generating the log string
2. Helper functions for logging object properties to support packet logging

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[sbSteveK]

Just griping to the void (you) that MQTT5 spec names this flag Clean Start and MQTT3.1.1 names it Clean Session