ShutdownDelay should apply on shutdowns and "healthy" restarts

[schmichael]

Background

The documentation for shutdown_delay makes it unclear under which scenarios the delay is applied. The behavior changed in Nomad 0.9.0:

• 0.8.7 - Delay applies only when stopping a task.
• 0.9.0 - Delay applies when stopping a task and when restarting a task.

PR #5957 would have tweaked the behavior further:

• Delay applies when stopping a task and when restarting a "healthy" task. Restarts due to failures - such as check_restart induced restarts - are not delayed.

Unfortunately PR #5975 was accepted over #5957 because it was a more conservative approach to fixing a critical bug (#5395), and the accepted PR lacks the shutdown delay improvement.

Enhancement

• Shutdown delay should not apply to unhealthy restarts like check_restart.
• Shutdown delay's documentation should be updated to reflect its restart impact.