Disable min_bytes_to_use_mmap_io by Default

Why it matters

This change addresses performance and stability concerns by preventing automatic use of memory-mapped IO for small byte sizes, which may not be efficient or beneficial in many scenarios.

How to use it

The min_bytes_to_use_mmap_io setting is now disabled by default. Users can manually enable or adjust this setting in the ClickHouse server configuration if memory-mapped IO is desired for specific workloads.