save_3d_views crashes kernel in Jupyter

[kroq-gar78]

If running in Jupyter, cortex.export.save_3d_views causes the kernel to restart after completing. (I have not been able to reproduce this in an IPython or plain Python shell.) Minimum example below:

import tempfile, cortex, cortex.export
vol = cortex.Volume.random(subject='S1', xfmname='fullhead')
viewer_params={'open_browser': True, 'autoclose': True}
filenames = cortex.export.save_3d_views(
    vol, base_name=tempfile.mkdtemp(), list_angles=['left'],
    list_surfaces=['inflated'], viewer_params=viewer_params)

This is a long-standing issue I've had, which I've worked around by editing save_3d_views to return the web handle as well. So my hunch is that there's some race condition between the teardown of the web server and the completion of the notebook cell.

[mvdoc]

I think it's something to do with tornado. See also
#351