mara's review on #2436: no submit-await-submit composition, even
server-side. Adds Template::GracefulRestart (Signal -> Drain ->
StopForUpdate -> Reconcile, wanted=Up) mirroring how Restart already
does StopForUpdate -> Reconcile, plus submit::graceful_restart and
templates::graceful_restart. handle_restart_scoped now submits exactly
one DAG per agent up front for both the graceful and non-graceful
case -- no await_dags in the loop anymore.