core: Use O_NOATIME to open metadata

We really don't need atime for metadata, it's just a speed hit.

src/libotutil/ot-unix-utils.h

@@ -59,6 +59,8 @@ gboolean ot_unix_fdatasync (int fd, GError **error);
 
 gboolean ot_unix_close (int fd, GError **error);
 
+gboolean ot_unix_open_noatime (const char *path, int *out_fd, GError **error);
+
 G_END_DECLS
 
 #endif