downloadFile has a memory leak.

I have a huge amount of files (hundreds of thousands) that need to download, but the file size is small, from a few to hundreds KB, so I use a loop to do it. And I watched the Perf Monitor, the memory usage are keeping increasing.
I searched the issues about the memory leak and found someone fixed [the upload memory leak issue](https://github.com/itinance/react-native-fs/pull/502), so is there someone who can help to check this situation existing in `downloadFile`? thank you in advance.