FILE_PING registers a JVM shutdown hook in init() but doesn't keep a reference to it, so the hook cannot be removed when the protocol stops.

• The shutdown hook to remain registered even after the channel is closed
• Memory leaks when channels are repeatedly created and destroyed - Thread is anonymous class and keeps reference to the stack via up/down protocol references
• Errors when closing gracefully and then shutdown hook operates on closed resources

This also affects subclasses such as JDBC_PING, S3_PING, etc.