add persistent goroutine to pullerGroup tomb

This avoids runtime panic from calling Go after all goroutines have returned.

Also, we need to kill the pullerGroup tomb in Stop() to tell the pullerGroup
that it's time to shut down.

internals/overlord/logstate/gatherer.go

@@ -230,6 +230,7 @@ func (g *logGatherer) Stop() {
 	// Kill the main loop once either:
 	// - all the pullers are done
 	// - timeoutMainLoop has passed
+	g.pullers.tomb.Kill(nil)
 	select {
 	case <-g.pullers.Done():
 		logger.Debugf("gatherer %q: pullers have finished", g.targetName)

internals/overlord/logstate/gatherer_test.go

@@ -122,7 +122,7 @@ func (s *gathererSuite) TestGathererShutdown(c *C) {
 	}()
 
 	select {
-	case <-time.After(20 * time.Millisecond):
+	case <-time.After(100 * time.Millisecond):
 		c.Fatalf("timeout waiting for gatherer to tear down")
 	case <-hasShutdown:
 	}

internals/overlord/logstate/puller.go

@@ -76,6 +76,10 @@ func newPullerGroup(targetName string) *pullerGroup {
 		targetName: targetName,
 		pullers:    map[string]*logPuller{},
 	}
+	pg.tomb.Go(func() error {
+		<-pg.tomb.Dying()
+		return nil
+	})
 	return pg
 }