list: support getting partial lists?

the list of all object ids in the backend / in the store could get rather long.

guess this needs to sort the IDs, so that we have a stable order?
maybe sorting each directory by itself is already good enough.

update:

backend: list is not that long as we only do flat single-directory listings and we usually use nesting to avoid too many items in a single directory.

store: does the recursion, so result is overall larger. but is a generator yielding single items, so no big lists are being built.