Add __next__(), next() to ResultProxy

Add __next__(), next() to ResultProxy

Added ``__next__()`` and ``next()`` methods to :class:`.ResultProxy`,
so that the ``next()`` builtin function works on the object directly.
:class:`.ResultProxy` has long had an ``__iter__()`` method which already
allows it to respond to the ``iter()`` builtin.   The implementation
for ``__iter__()`` is unchanged, as performance testing has indicated
that iteration using a ``__next__()`` method with ``StopIteration``
is about 20% slower in both Python 2.7 and 3.6.

Change-Id: I70569a4c48ad85a3c21a7ad422f270a559926cfb
Fixes: #4077