emit TaskRestartSignal event on vault restart

When Vault token expires and task is restarted, emit `TaskRestartSignal`
similar to v0.8.7

client/allocrunner/taskrunner/task_runner_test.go

@@ -1420,21 +1420,27 @@ func TestTaskRunner_VaultManager_Restart(t *testing.T) {
 			return false, fmt.Errorf("no events yet")
 		}
 
-		// TODO: check for RestartSignal too - 0.8 sent that event, but not 0.9
-		foundRestarting := false
+		foundRestartSignal, foundRestarting := false, false
 		for _, e := range state.Events {
-			if e.Type == structs.TaskRestarting {
+			switch e.Type {
+			case structs.TaskRestartSignal:
+				foundRestartSignal = true
+			case structs.TaskRestarting:
 				foundRestarting = true
 			}
 		}
 
+		if !foundRestartSignal {
+			return false, fmt.Errorf("no restart signal event yet: %#v", state.Events)
+		}
+
 		if !foundRestarting {
 			return false, fmt.Errorf("no restarting event yet: %#v", state.Events)
 		}
 
 		lastEvent := state.Events[len(state.Events)-1]
 		if lastEvent.Type != structs.TaskStarted {
-			return false, fmt.Errorf("expected last event to be task restarting but was %#v", lastEvent)
+			return false, fmt.Errorf("expected last event to be task starting but was %#v", lastEvent)
 		}
 		return true, nil
 	}, func(err error) {

client/allocrunner/taskrunner/vault_hook.go

@@ -254,7 +254,7 @@ OUTER:
 			case structs.VaultChangeModeRestart:
 				const noFailure = false
 				h.lifecycle.Restart(h.ctx,
-					structs.NewTaskEvent(structs.TaskRestarting).
+					structs.NewTaskEvent(structs.TaskRestartSignal).
 						SetDisplayMessage("Vault: new Vault token acquired"), false)
 			case structs.VaultChangeModeNoop:
 				fallthrough