object_store: Make iter_tree_contents depth-first.

object_store: Make iter_tree_contents depth-first.

This mimics the behavior of os.walk as well as common tools like find
and ls -R.

Memory usage may also be better, as directory trees tend to have a
higher branching factor than depth. (Note that the overhead of Python
stack frames may make this not true in many cases, and I haven't run
any benchmarks.)