KIP-20 — Enable log preallocate to improve consume performance under windows and some old Linux file system

Accepted Broker

KIP-20 adds a `log.preallocate` broker config that pre-allocates log segment files at creation time (using the full `log.segment.bytes` size) rather than growing them incrementally via appends. This reduces filesystem fragmentation on Windows and older Linux filesystems (ext2/ext3), improving consume performance for sequential reads.

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