Modify how we skip unneeded front images from non-CAA sources

Instead of looping and modify the try list, just check in _walk()
and continue.
Improve debugging messages (no more silent removal in debug mode).
try_list -> queue + fix up few comments
Hide underlying queue/list using a bunch of new methods

- _append_image() -> _queue_put()
- new _queue_get()
- new _queue_empty()
- new _queue_new()