perf improvements in misc_helpers functions

[ZhidongPeng]

• Update json_write_to_file to use BufWriter to reduce system calls and improve performance
• Introduced new json_write_to_file_async using tokio::fs, provides better throughput in async contexts
• At get date_time string functions, started to use static LazyLock format descriptors. The format strings are now parsed once on first use and reused for all subsequent calls