Feature 6.2: Concurrency and Parallelism

[ja7ad]

Implement advanced concurrency and parallelism techniques for high-performance MQTT. Use worker pool pattern for message processing, lock-free queues and ring buffers for inter-goroutine messaging, and context cancellation for graceful shutdown.

Sub-issues:

• Implement worker pool pattern for message processing
• Introduce lock-free queues and ring buffers for inter-goroutine messaging
• Use context cancellation and graceful shutdown signaling