feat: add Close() method to RedHub server

[gitsrc]

Add a Close() method to allow graceful shutdown of the RedHub server. This addresses issue #14 and provides a similar API to tidwall/redcon.

Changes:

• Add mu, addr, and running fields to RedHub struct for server state
• Add Close() method that gracefully shuts down the server
• Modify ListenAndServe to track server state
• Add unit tests for Close() functionality
• Add integration test for Close() method

The Close() method:

• Returns error if server is not running
• Calls gnet.Stop() to gracefully shutdown
• Is thread-safe with mutex protection
• Can be called from any goroutine