Update comments to reflect that multipart upload URLs are not actually
used for upload traffic - uploads go directly to volume servers.
Key clarifications:
- genPartUploadUrl: Filer address is parsed out, only path is used
- CompleteMultipartUpload Location: Informational field per AWS S3 spec
- Actual uploads bypass filer proxy and go directly to volume servers
The filer address in these URLs is NOT a HA concern because:
1. Part uploads: URL is parsed for path, upload goes to volume servers
2. Location URL: Informational only, clients use S3 endpoint
This addresses the observation that S3 uploads don't go through filers,
only metadata operations do.
Chris Lu
3aa9b5f4fe