implement alloc runner task restart hook

[tgross]

Fixes #9176

Most allocation hooks don't need to know when a single task within the
allocation is restarted. The check watcher for group services triggers the
alloc runner to restart all tasks, but the alloc runner's Restart method
doesn't trigger any of the alloc hooks, including the group service hook. The
result is that after the first time a check triggers a restart, we'll never
restart the tasks of an allocation again.

This commit adds a RunnerTaskRestartHook interface so that alloc runner
hooks can act if a task within the alloc is restarted. The only implementation
is in the group service hook, which will force a re-registration of the
alloc's services and fix check restarts.

[shoenig]

Does this mean the warning can be removed from https://www.nomadproject.io/docs/job-specification/check_restart ?

[carmstrong]

@tgross We see this was flagged as part of the 1.0.3 milestone, but the release notes suggest 1.0.3 is only security fixes. Should we expect this in 1.0.4?

[tgross]

The 1.0.3 milestone got re-named to 1.0.4 once the 1.0.3 release went out. Because that was a security patch we weren't able to rename the milestone ahead of disclosing it. So this will go out in 1.0.4.

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