Slow-filling buffers

[voegtlel]

Filling the shuffle buffer (and packing buffer) should have an option to fill up slowly (e.g. loading two items per one yielded item until full) instead of filling up the whole buffer at startup.
Also, for restoring the buffer, that could potentially happen like 2 at a time (i.e. the item to be yielded and one other item of the buffer).