Feature 5.2: Load Balancing and Scalability

[ja7ad]

Implement load balancing and scalability features for high-availability MQTT deployment. Integrate SO_REUSEPORT and multi-core listener optimization, message batching and flow control across nodes, and shared subscription balancing across cluster nodes.

Sub-issues:

• Integrate SO_REUSEPORT and multi-core listener optimization
• Implement message batching and flow control across nodes
• Support shared subscription balancing across cluster nodes