Research the possibility of making the ExternalFileManager faster by caching the CachingDocumentFile inside the ExternalFile and passing them down with every other operation

[K1rakishou]

Because right now we use toDocumentFIle() internal method every time we want to get a DocumentFile (or rather CachingDocumentFile) for an ExternalFile and for that we do FindDeepFile which is really slow. So there is a possibility that the speed can be really boosted up by caching the resulting file.