NanoMQ v0.6.6 更新日志

1. Update the embedded MQTT SDK (nanosdk) to 0.3.
2. Refactoring NNG_PIPE system for better session managing.
3. Optimize clean session implementation.
4. Add nano_bench tool for benchmarking.
5. Fix MQTT V5 compliance. (CONNACK :User Property & Session Expiry & Flow control)
6. Add new HTTP API for statistical report & update configuration remotely
7. Fix other minor bugs of 0.6.0 regarding UNSUB & PINGREQ & Shared-subscription etc..

LTS of 0.6

Mar 21st Update : Fix #445 Mar 30th Update : Sync with 0.6.7 & fix SDK https://github.com/nanomq/NanoSDK/issues/34