Clarified shared reference between Session and IOManager.

Session holds a reference to an IOManager.

Part of Session.__init__():

OLD:

    self._io_manager = iotools.IOManager(self)

NEW:

    self._io_manager = iotools.IOManager(session=self)

The IOManager initializer signature is IOManager.__init__(client=None,
session=None). So the old form was mistakening initializing the IOManager with
the incorrect session.