Allow the kernel manager to shut down gracefully in _async_cleanup_kernel about nbclient

[golf-player]

CC: @kevin-bates

[kevin-bates]

Yes, I think the nbclient shutdown sequence should allow the kernel manager to perform a graceful shutdown (now=False). This allows implementations of request_shutdown and finish_shutdown to perform actions and finish_shutdown will trigger the equivalent of an immediate shutdown via _kill_kernel() anyway.

With remote kernels, the actions necessary during graceful shutdowns include enlisting the help of associated resource managers (YARN, k8s, etc.) to properly stop the "application" (i.e., kernel process) whereas the immediate shutdown, while still enlisting the resource manager, uses more of a brute force termination tactic (e.g., issues KILL via REST API on YARN). Both approaches are necessary and both occur via the single shutdown call (when now = False).

[golf-player]

I've made #64