Implement Automatic Subscription Cleanup with Client Lifecycle Tracking

[tonatoz]

Problem Solved

Previously, WebSocket subscriptions in Dshackle lacked proper cleanup mechanisms, leading to potential memory leaks and resource waste. When clients disconnected, subscriptions would remain active upstream even when no longer needed, causing:

• Accumulation of orphaned subscriptions
• Unnecessary resource consumption
• Potential memory leaks
• No tracking of which clients were using which subscriptions

New Architecture Overview

This PR introduces a comprehensive client lifecycle tracking system for WebSocket subscriptions with automatic cleanup:

Phase 1: Reference Counting Infrastructure

• SubscriptionHolder: Manages individual subscriptions with client tracking instead of simple reference counting
• SubscriptionCleanup: Interface for handling cleanup operations when subscriptions are no longer needed
• Updated GenericIngressSubscription to support proper subscription lifecycle management

Phase 2: Client Lifecycle Tracking

• SubscriptionClient: Interface representing a client with unique identification and disconnect callbacks
• DefaultSubscriptionClient: Default implementation with automatic cleanup on disconnect
• ClientAwareSubscriptionConnect: Wrapper that monitors client connection state and triggers cleanup on disconnect/cancel/error
• Enhanced SubscriptionHolder to track individual clients rather than just reference counts

Phase 3: Comprehensive Testing

• Added extensive test coverage for the new subscription management system
• Tests verify proper client tracking, cleanup behavior, and edge cases
• Updated existing tests to work with the new architecture

Key Features

1. Automatic Cleanup: Subscriptions are automatically cleaned up when the last client disconnects
2. Client Tracking: Each client gets a unique ID and is tracked individually
3. Robust Disconnect Handling: Handles disconnection via cancellation, completion, errors, and manual disconnect
4. Resource Management: Prevents memory leaks by properly unsubscribing from upstream when no clients remain
5. Monitoring Support: Provides subscription statistics and client information for monitoring
6. Thread Safety: All operations are thread-safe using concurrent data structures

Implementation Details

• Client ID Generation: Unique IDs based on timestamp, thread ID, and random component
• Subscription Sharing: Multiple clients can share the same subscription (topic + params)
• Graceful Cleanup: Automatic unsubscription from upstream using appropriate methods (e.g., eth_unsubscribe, accountUnsubscribe)
• Error Handling: Robust error handling during cleanup operations with proper logging