New method to calculate average round trip time.

Replace 5-sample moving average with exponentially weighted average,
required by the upcoming Server Selection Spec. Move responsibility for
tracking the average from ServerDescription to Monitor. Now the
ServerDescription only knows the average at the time it was created.
Delete an ancient note about MongoDB 1.5.
Make Pool.get_socket a context manager.

SocketInfo itself is no longer a context manager.
Simplifies exhaust cursors and exception handling,
and avoids a confusing weakref.
Dead code.
Delete an ancient FAQ about the Year 2038 Problem.
Rely on standard json module.

Now that we've dropped Python 2.4 and 2.5, we know the standard json module is
available.