Before this commit if a task had 2 checks cause restarts at the same
time, both would trigger restarts of the task! This change removes all
checks for a task whenever one of them is restarted.
@dadgar made the excellent observation in #3105 that TaskRunner removes
and re-registers checks on restarts. This means checkWatcher doesn't
need to do *any* internal restart tracking. Individual checks can just
remove themselves and be re-added when the task restarts.
