This task previously read all images file's EXIF data every 60s, which
leads to a very large CPU usage for seaf_fuse.
The obvious fix of checking for modifications times and only read the
change files does not work with seaf_fuse, because stat is not cheaper
than actually opening and reading the file.
So instead, we now only read the file list and the modification time of
the image directory itself and update the image metadata only if there
are changes. We also update the metadata at least once every hour to
detect modified files on normal filesystems.
Interestingly, seaf_fuse will update the mtime of a directory if any file
in that directory is changed, so the logic implemented in this commit
will actually detect image changes with seaf_fuse instantly.
