This change brings public data-types such that
we can ask projects to implement gateway projects
externally than maintaining in our repo.
All publicly exported structs are maintained in object-api-datatypes.go
completePart --> CompletePart
uploadMetadata --> MultipartInfo
All other exported errors are at object-api-errors.go
Previously ID/ETag from backend service is used as is which causes
failure on s3cmd like tools where those tools use ETag as checksum to
validate data. This is fixed by prepending "-1".
Refer minio/mint#193minio/mint#201
Verify() was being called by caller after the data
has been successfully read after io.EOF. This disconnection
opens a race under concurrent access to such an object.
Verification is not necessary outside of Read() call,
we can simply just do checksum verification right inside
Read() call at io.EOF.
This approach simplifies the usage.
