Turn off md5sum optionally if content-md5 is not set (#7609)

This PR also brings --compat option to run MinIO in strict
S3 compatibility mode, MinIO by default will now try to run
high performance mode.

cmd/fs-v1-multipart.go

@@ -504,10 +504,7 @@ func (fs *FSObjects) CompleteMultipartUpload(ctx context.Context, bucket string,
 	}
 
 	// Calculate s3 compatible md5sum for complete multipart.
-	s3MD5, err := getCompleteMultipartMD5(ctx, parts)
-	if err != nil {
-		return oi, err
-	}
+	s3MD5 := getCompleteMultipartMD5(parts)
 
 	partSize := int64(-1) // Used later to ensure that all parts sizes are same.