Relax flush to disk in disk segment creation. - #171

Summary

• Relax compressed random-access device flushing during disk segment creation
• Change per-compressed-block flushes from durable Flush(true) to buffered Flush(false)
• Preserve durability at the publish point: SealDevice still writes terminal block metadata and durably flushes the complete compressed file
• Document the reasoning inline so the per-block flush behavior is clear