Provide special row proxies for count and index

Provide special row proxies for count and index

The Python ``namedtuple()`` has the behavior such that the names ``count``
and ``index`` will be served as tuple values if the named tuple includes
those names; if they are absent, then their behavior as methods of
``collections.abc.Sequence`` is maintained. Therefore the
:class:`_result.Row` and :class:`_result.LegacyRow` classes have been fixed
so that they work in this same way, maintaining the expected behavior for
database rows that have columns named "index" or "count".

Fixes: #6074
Change-Id: I49a093da02f33f231d22ed5999c09fcaa3a68601