s3: fix excess memory usage when using versions

Before this change, we were taking the version ID straight from the
XML blob returned by the SDK and thus pinning the XML into memory
which bulked up the average memory per object from about 400 bytes to
4k.

Copying the string fixes the excess memory usage.

backend/s3/s3.go

@@ -3093,7 +3093,7 @@ func (f *Fs) newObjectWithInfo(ctx context.Context, remote string, info *s3.Obje
 		o.setMD5FromEtag(aws.StringValue(info.ETag))
 		o.bytes = aws.Int64Value(info.Size)
 		o.storageClass = stringClonePointer(info.StorageClass)
-		o.versionID = versionID
+		o.versionID = stringClonePointer(versionID)
 	} else if !o.fs.opt.NoHeadObject {
 		err := o.readMetaData(ctx) // reads info and meta, returning an error
 		if err != nil {