Fix indefinite hangs on non-EC2 network paths

[nkk0]

coldsnap uploads can hang indefinitely when running from outside AWS. A small fraction of PutSnapshotBlock requests complete the TCP handshake but never receive an HTTP response. The AWS Rust SDK does not set read_timeout, operation_attempt_timeout, or operation_timeout by default, so these requests block the worker forever.

Addresses: #437

Changes:

• Set SDK timeouts in build_client_config(): read_timeout 12s, operation_attempt_timeout 20s, operation_timeout 120s. These apply to both the upload and download CLI paths.

• Set SDK max_attempts to 1. coldsnap has its own per-block retry loop with backoff; layering SDK retries on top produced up to 36 attempts per block with no coordinated timeout.

• Reduce block retry count from 12 to 5. With bounded per-attempt timeouts, fewer retries are needed.

• Add --workers flag (default 64) to configure concurrent upload workers. Reject --workers 0 since for_each_concurrent treats 0 as unlimited.

• Add --client-shards flag (default 1) to create N independent EbsClient instances for uploads. Blocks are distributed by index (block_index % N). This is opt-in; the default preserves the existing single-client behavior. In our testing, --client-shards 8 with 64 workers improved the per-block latency profile on high-latency paths.

• Log a latency histogram at INFO level after the upload completes (<250ms, 250-500ms, 500ms-1s, 1-2s, 2-5s, >5s, plus error count).

• Log per-block warnings on failure with block index, attempt number, elapsed time, and error. Previously failures were logged at DEBUG with no context.

Tested with 18 consecutive uploads of a 4.1 GiB image from GitHub Actions runners across Virginia, Wyoming, and other Azure regions. All 18 succeeded (20-51s depending on config and runner location). Stock coldsnap on the same paths hung indefinitely or took 5-20+ minutes.

[jmt-lab]

Thank you for this contribution. This looks good to me.

[jmt-lab]

This is a good catch here

[jmt-lab]

Also we do suggest you setup ssh key signing for your git commits to help verify your commits. This is not a hard requirement for contributions externally but generally a good thing to do: https://docs.github.com/en/authentication/managing-commit-signature-verification/about-commit-signature-verification#ssh-commit-signature-verification

[nkk0]

Also we do suggest you setup ssh key signing for your git commits to help verify your commits. This is not a hard requirement for contributions externally but generally a good thing to do: https://docs.github.com/en/authentication/managing-commit-signature-verification/about-commit-signature-verification#ssh-commit-signature-verification

Done! Sorry about that

[jmt-lab]

Thank you for your contribution!