xl: Use random UUID during complete multipart upload (#7527)

One user has seen this following error log:

API: CompleteMultipartUpload(bucket=vertica, object=perf-dss-v03/cc2/02596813aecd4e476d810148586c2a3300d00000013557ef_0.gt)
Time: 15:44:07 UTC 04/11/2019
RequestID: 159475EFF4DEDFFB
RemoteHost: 172.26.87.184
UserAgent: vertica-v9.1.1-5
Error: open /data/.minio.sys/tmp/100bb3ec-6c0d-4a37-8b36-65241050eb02/xl.json: file exists
       1: cmd/xl-v1-metadata.go:448:cmd.writeXLMetadata()
       2: cmd/xl-v1-metadata.go:501:cmd.writeUniqueXLMetadata.func1()

This can happen when CompleteMultipartUpload fails with write quorum,
the S3 client will retry (since write quorum is 500 http response),
however the second call of CompleteMultipartUpload will fail because
this latter doesn't truly use a random uuid under .minio.sys/tmp/
directory but pick the upload id.

This commit fixes the behavior to choose a random uuid for generating
xl.json

cmd/xl-v1-object.go

@@ -103,6 +103,9 @@ func (xl xlObjects) CopyObject(ctx context.Context, srcBucket, srcObject, dstBuc
 
 		tempObj := mustGetUUID()
 
+		// Cleanup in case of xl.json writing failure
+		defer xl.deleteObject(ctx, minioMetaTmpBucket, tempObj, writeQuorum, false)
+
 		// Write unique `xl.json` for each disk.
 		if onlineDisks, err = writeUniqueXLMetadata(ctx, storageDisks, minioMetaTmpBucket, tempObj, metaArr, writeQuorum); err != nil {
 			return oi, toObjectErr(err, srcBucket, srcObject)