[broker] add a persistent database for aedes server

[abhijithvijayan]

You can use Mongo, Redis and LevelUp for persistence storage (mostly for the offline messages feature) that ensures Quality of Service (QoS)

[abhijithvijayan]

• https://thenewstack.io/messaging-reliability-persistence-mqtt/
• https://www.hindawi.com/journals/js/2018/1987931/
• https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6696184/
• https://dzone.com/articles/internet-things-mqtt-quality