Make closing jupyter client faster

[impact27]

When calling stop on a HBChannel, one has to wait until the sleep command ends. This would allow a much faster shutdown time.

[ccordoba12]

@minrk, @takluyver, we need this in Spyder because we set a very long time_to_dead time for the heartbeat channel to avoid too many empty prompts in qtconsole when the kernel takes too much time to start (which is a common problem for us, specially on Windows).

Thanks for your help!

[minrk]

closing and reopening to retrigger tests. 👍 from me.

[ccordoba12]

Thanks a lot Min!