VerneMQ Broker MQTTv5 support

[Lars Hesel Christensen]

Hi everyone!

We just wanted to let you know we've released an early preview of
MQTTv5 support in the VerneMQ MQTT broker and it is available on
Github on the `mqtt5-preview` branch:
https://github.com/erlio/vernemq/tree/mqtt5-preview. As the VerneMQ
broker itself this is of course also open source, licensed under the
Apache Version 2 License.

At this point we're actively working on getting the final bits
implemented and fix bugs etc. We'd be grateful for feedback if you try
it out.

After some maturation and polishing we'll merge the MQTTv5
implementation into VerneMQ where it'll be available as a beta-feature
until we're sure it's production ready.

Currently supported features are:

- All MQTTv5 frames are implemented.
- MQTTv5 enhanced authentication and reauthentication.
- Publishes with payload format indicator, response topic, correlation data,
  user properties and content type.
- Message expiration.
- Delayed last will and testament.
- Retained messages.
- Shared subscriptions.
- Session expiration interval.
- Request/response using 'response topic' and 'correlation data' properties.
- Client to broker topic aliases.
- MQTTv5 and older prototocols can be enabled at the same time (set
  `allowed_protocol_versions=3,4,5` on the listener to enable respectively MQTT
  v3.1, 3.1.1 and 5.0).

We're looking forward to seeing MQTT 5.0 in the wild as we are excited
about the use cases the new features enable.

The VerneMQ engineering team,
Lars