Implement a Thread-Safe Pub-Sub Service

Implement a simple server-side publish-subscribe system using Python's standard `threading` library. Requirements: - Multiple publisher threads may publish messages concurrently. - Multiple subscriber threads may subscribe to topics concurrently. - A subscriber can subscribe to and unsubscribe from a topic. - When a message is published to a topic, all currently subscribed subscribers should receive the message. - The implementation must avoid race conditions and deadlocks. - Use Python threading primitives such as `Lock`, `RLock`, `Condition`, `Event`, and thread-safe queues where appropriate. - Discuss delivery semantics, shutdown behavior, and how to avoid one slow subscriber blocking all publishers.