Minor parallelism improvements

[mcandre]

Can we please provide a faster default for the number of parallel file transfers? I think a default setting of 4 concurrent file transfers would really improve out of the box performance.

• 4 matches similar concurrency settings for FTP clients.
• 4 integrates well with modern CPU core count configurations.
• 4 has low risk of triggering transfer errors in Kubernetes.
• 4 has low risk of triggering transfer errors in S3.

Also, I wonder if the value of 0 for the concurrency setting should be rejected by skbn. This is because full concurrency of all files at once, tends to trigger transfer cooldown errors. Because file trees with very many files, will basically create a denial of service attack as far as S3 sees.