Address race condition between shutdown_kernel and restarter

[kevin-bates]

There is a window in which the auto-restarter logic (that detects unexpectedly terminated kernels) can restart a shutdown kernel - whose termination was intended.

Although rare, this has been seen when a Notebook server is terminated with running kernels, at which time it issues shutdown requests for each kernel - while, at the same time, but after the kernel process has terminated, the restarter detects the kernel process's absence and restarts the kernel.

By indicating that the kernel manager has started a (graceful) shutdown of the kernel, the restarter can better-determine the difference between an intended and unintended shutdown - skipping the kernel restart in case of the former.

Resolves #606

[Carreau]

that looks good to me, let's get that in.