list: support getting partial lists?

[ThomasWaldmann]

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.

[ThomasWaldmann]

#15 implements sorting for .list.

if we have a sorted (or at least stable, not random) list order, we can use e.g. the last list member for continuing the listing with another .list call that lists everything AFTER the given member.

[ThomasWaldmann]

There is also a related .list issue in borg and the interaction between borg and borgstore is rather a slow (but at least correct) hack right now.