Multiple instances of MQTT Agent are unsafe due to statically allocated memory

[stephenglynch]

The library seems otherwise well designed to allow multiple instances of MQTT Agents e.g. to support simultaneous connections to more than one broker. However, there are two statically allocated arrays (v1.3.1) used to handle publishes with QoS > 0 that make multiple instances unsafe as the arrays will be reinitialised and the memory effectively shared between instances.

The offending statically allocated arrays are in the function MQTTAgent_Init, named pIncomingPublishRecords and pOutgoingPublishRecords.

It would seem reasonable to move these arrays into MQTTAgentContext_t which should make the library safe.

[aggarg]

The MQTT Agent's architecture was not designed for multiple concurrent instances. However, your proposed modification seems reasonable. Would you be interested in submitting a PR to implement this enhancement?

[ActoryOu]

I'm closing this issue because #132 has been merged.