If Future.cancel() returns False this does *not* mean that the
cancellation failed, so we should not check for this. Instead
we just cancel all the outstanding futures and wait for them to
die.

If the runner instantiates its own executor, then it is also
responsible for calling 'shutdown'. This was the cause of a
bug in the notebook integration where kernels would fail to die
if ProcessPoolExecutors were not properly shut down. This is probably
a bug in IPython, but we fix our usage here also.

Better loss improvement

See merge request !6