[step_runner/subproc] Improve the recipe engine's kill routine.
[step_runner/subproc] Improve the recipe engine's kill routine.

This will allow killed processes the chance to gracefully shut down (with
a hard-coded 30s grace period for now).

R=tandrii@chromium.org

Bug: 910369
Recipe-Nontrivial-Roll: skia
Change-Id: I5a71208a6c933bf33fa97b08cd6cdc2d3ebef70a
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/recipes-py/+/1642344
Reviewed-by: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
Commit-Queue: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>

luci/recipes-py,luci/recipes-py