immutable/filenode.py: put off DownloadStatus creation until first read() call

immutable/filenode.py: put off DownloadStatus creation until first read() call

This avoids spamming the "recent uploads and downloads" /status page from
FileNode instances that were created for a directory read but which nobody is
ever going to read from. I also cleaned up the way DownloadStatus instances
are made to only ever do it in the CiphertextFileNode, not in the
higher-level plaintext FileNode. Also fixed DownloadStatus handling of read
size, thanks to David-Sarah for the catch.