NanoMQ v0.6.8 更新日志

NanoMQ 0.6.8 change log:

1. Support SQLite as QoS message persistence option
2. MQTT V5 new Features: 2.1 MQTT V5 Subscription Identifier 2.2 Message expiry interval 2.3 Retain as Published & Retain handling 2.4 MQTT V5 Topic Alias
3. WebHook system (currently support CONNACK & PUBLISH & DISCONNECT)
4. New module named nngproxy : convert NNG message to MQTT message.
5. Fix Windows build
6. Fix MQTT V5 compatibility on TLS/WebSocket port.

May 9th: republish 0.6.8 for https://github.com/emqx/nanomq/issues/559 & https://github.com/emqx/nanomq/issues/566