swarming: properly handle task with sleeping bot
swarming: properly handle task with sleeping bot

Request the bot to terminate the task early if it had been detected as MIA.

Otherwise you get a BOT_DIED task that runs for a long time after it was
marked as BOT_DIED, which is super confusing.

R=747cf52e0cef2e3e9371802a2d344b6c87126a4b@chromium.org

Bug: 863150
Change-Id: I4926e3b0403356603662ed50d92fef49f6e248f0
Reviewed-on: https://chromium-review.googlesource.com/1135664
Commit-Queue: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
Reviewed-by: Quinten Yearsley <747cf52e0cef2e3e9371802a2d344b6c87126a4b@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py