walk_object: rewrite as nonrecursive

walk_object: rewrite as nonrecursive

Given a deep enough bup tree the current recursive version can exceed
the default Python stack limit, resulting in an error like this:

  RuntimeError: maximum recursion depth exceeded in cmp

Rewrite the function as an iteration, and explicitly manage the pending
items as a list on the heap.

Thanks to axion for reporting the problem.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>