feat: Add worker pool for concurrent DNS request handling

[127ewyo3]

feat: Add worker pool for concurrent DNS request handling

This commit introduces a goroutine worker pool to optimize the handling of incoming DNS requests. Instead of launching a new goroutine for every single request, a fixed number of worker goroutines are started, and incoming requests are placed into a buffered channel.

This approach provides the following benefits:

• Resource Control: It limits the maximum number of concurrent requests being processed, preventing the server from becoming overwhelmed during traffic spikes.
• Improved Stability: By bounding concurrency, it ensures predictable resource usage, reducing the risk of resource exhaustion on the server (e.g., too many open files, excessive memory usage).
• Reduced Overhead (for very high-frequency, short-lived tasks): While Go's goroutines are lightweight, a worker pool can offer a slight performance edge by amortizing the cost of goroutine creation and garbage collection over many requests.

The core changes are in handler.go:

• A new request struct is defined to hold the network type, ResponseWriter, and dns.Msg.
• A buffered channel, requestChan, is added to the GODNSHandler struct to serve as a work queue.
• The NewHandler function is modified to initialize the worker pool by starting a predefined number of worker goroutines.
• The DoTCP and DoUDP methods are simplified to push incoming requests onto the requestChan.
• The original request processing logic from DoTCP and DoUDP is moved into a new handleRequest method, which is executed by the worker goroutines.

This change is a production-level optimization that enhances the server's resilience and resource management under high load. The server.go file remains unchanged as it correctly delegates request handling to the handler instance.