Refactor stats.lua to add pagination support

[veteranbv]

This update enhances the stats.lua script by implementing pagination, enabling users to view and analyze larger datasets efficiently. Several ASF projects have monthly activity exceeding the 5000-message limit previously imposed. With pagination, users can navigate through data in manageable chunks, making the script more scalable and suitable for high-volume mailing lists.

Additionally:

• Removed duplicate variable declarations for dhh and total_hits to improve code maintainability.
• Refactored query logic to handle offsets and scrolling for large result sets, ensuring compatibility with Elasticsearch's MAX_RESULT_WINDOW.

These changes improve usability for larger ASF projects while maintaining backward compatibility for smaller datasets.