Merge bzr.dev 4032. Resolve the new streaming fetch.
XXX: We cheat a bit for CHK fetching. CHK serializers happen to still have legacy 'read_inventory_from_string' and 'write_inventory_to_string' functions that convert the paged representation to a single-string XML representation.
So when converting between formats, we just go down to the whole-inventory XML form.
At least it works for now. Even if it is grossly innefficient.