MQTT ID suggestion

[pmeskoni]

Currently the next available mqtt_id is stored in mg_mgr
if (++c->mgr->mqtt_id == 0) ++c->mgr->mqtt_id;
id = c->mgr->mqtt_id;

The mqtt_id field should be moved to the mqtt_connection structure.
There is no system-wide uniqueness requirement, only unique IDs are required for a given connection.
In high throughput situations, with multiple connections to MQTT brokers, there is a possibility for duplicate IDs if there are too many pending QoS 2 requests in-flight.

[scaprile]

Please see our documentation, and follow the guidelines in our tutorials.

Mongoose is single threaded, our tutorials instruct to catch the ID as soon as the message is published and handled by the user.
https://mongoose.ws/documentation/tutorials/mqtt/mqtt-client/#sending-qos-1-messages

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