File change detection: Also look at file sizes.

See #2167 and #2169.

* csync_detect_update: Reupload a file if the sizes differ, similar
  to how we reupload if the mtimes differ.
* PropagateUploadFileQNAM::slotPutFinished(): Abort an upload if the
  size changed, similar to how we abort an upload if the mtimes changed.