500 when performing prefix listing ending with '\U0010ffff'

At least, on py3 -- I think py2 would tolerate it well enough?

Affects both account and container servers; the backends complain like

  Traceback (most recent call last):
    File "/vagrant/swift/swift/account/server.py", line 323, in __call__
      res = getattr(self, req.method)(req)
    File "/vagrant/swift/swift/common/utils.py", line 2026, in _timing_stats
      resp = func(ctrl, *args, **kwargs)
    File "/vagrant/swift/swift/account/server.py", line 265, in GET
      return account_listing_response(account, req, out_content_type, broker,
    File "/vagrant/swift/swift/account/utils.py", line 83, in account_listing_response
      account_list = broker.list_containers_iter(limit, marker, end_marker,
    File "/vagrant/swift/swift/account/backend.py", line 386, in list_containers_iter
      end_prefix = prefix[:-1] + chr(ord(prefix[-1]) + 1)
  ValueError: chr() arg not in range(0x110000)