fs/erasure: Rename meta 'md5Sum' as 'etag'. (#4319)

This PR also does backend format change to 1.0.1
from 1.0.0.  Backward compatible changes are still
kept to read the 'md5Sum' key. But all new objects
will be stored with the same details under 'etag'.

Fixes #4312

cmd/gateway-s3.go

@@ -295,7 +295,7 @@ func fromMinioClientObjectInfo(bucket string, oi minio.ObjectInfo) ObjectInfo {
 		Name:            oi.Key,
 		ModTime:         oi.LastModified,
 		Size:            oi.Size,
-		MD5Sum:          oi.ETag,
+		ETag:            oi.ETag,
 		UserDefined:     userDefined,
 		ContentType:     oi.ContentType,
 		ContentEncoding: oi.Metadata.Get("Content-Encoding"),
@@ -326,13 +326,13 @@ func (l *s3Gateway) PutObject(bucket string, object string, size int64, data io.
 	}
 
 	var md5sumBytes []byte
-	md5sum := metadata["md5Sum"]
+	md5sum := metadata["etag"]
 	if md5sum != "" {
 		md5sumBytes, err = hex.DecodeString(md5sum)
 		if err != nil {
 			return ObjectInfo{}, s3ToObjectError(traceError(err), bucket, object)
 		}
-		delete(metadata, "md5Sum")
+		delete(metadata, "etag")
 	}
 
 	oi, err := l.Client.PutObject(bucket, object, size, data, md5sumBytes, sha256sumBytes, toMinioClientMetadata(metadata))