change default 'shutdown_executor' to False

Shutting down executors that are passed in by the user is not a
good default. Previously this was useful because ipykernels would
not shut down correctly because of some weird interaction with open
'concurrent.futures.Executor's, but this has since been fixed.
2 jobs for v0.3.0 in 1 minute and 50 seconds (queued for 1 second)