If any file being processed was modified less than 15 seconds ago, wait until modified time plus 15 seconds and retry.

This should be fully compatible with "scan and process", "manual post
process", and automated post processing for SABnzbd downloads.

This prevents SB from trying to process files when they are still being
copied (as opposed to being moved which is usually instantaneous) or
unpacked into the TV download directory.

Previously, SB would interrupt the transfer by moving the partial
file to its final destination.