fix kernel can only restart once issue

[mofanke]

fix kernel can only resstart once issue
fix issue #689

[kevin-bates]

Hi @mofanke - thank you for the pull request. I was thinking the reset would occur at the start of the start_kernel() method (rather than its post-start). Did you try that and was it problematic?

[mofanke]

Hi, @kevin-bates I have tried the reset in start_kernel(), pre_start_kernel() and post_start_kernel() , all the three ways can git rid of the problem. "self.shutting_down = True" setted in _async_shutdown_kernel() just before "self.stop_restarter()" so I put "self.shutting_down = False" as the reset just before "self.start_restarter()" in _async_post_start_kernel().

shoud I update the PR , and set "self.shutting_down = False" in start_kernel()?

[kevin-bates]

Hi @mofanke - thanks for looking into the various solutions. In looking at the history of self.shuttting_down I must have inadvertently removed its reset to False when introducing kernel provisioners. It was previously located at the start of pre_start_kernel(), which makes the most sense to me since, at that point, it's no longer shutting down. As such, I think its best to place it in that same relative location. Thank you.

[kevin-bates]

LGTM - thank you @mofanke!