Add support for client persistence

[njh]

Types of storage for persistence:

• in memory
• flat file on disk
• Redis
• Relational Databases
  • SQLite
  • MySQL

I am not sure if it would be possible to use Memcache - I think a key quality would be to iterate through the persisted messages.

This is a pre-requisite for QoS 1 and 2.

[Vincent14]

Without QoS, what's the goal of this issue?

[njh]

Persistence would allow clients to act as if the server is there all of the time - responsibility for the message would be handed over to the MQTT library.

This means if the connection to the server dropped or the client application crashes, the library can deliver the messages when the connection is back up again.

If could also act as a buffer, if there was a lot of data/messages to send to the MQTT server.