chore(gridcache): log what each batch writes

A batch that never reaches the disk looks exactly like one that does: the
locators are in memory either way until the next restart, which is the one
moment the difference shows. The shutdown flush was the least observable of
all — it runs last, and nothing said whether it had written anything.

internal/gridcache/gridcache.go

@@ -186,6 +186,10 @@ func (s *Store) Flush() error {
 		s.requeue(batch)
 		return err
 	}
+	// Logged because a batch that never reaches the disk looks exactly like one
+	// that does: the locators are in memory either way until the next restart,
+	// which is the one moment the difference shows.
+	s.logf("gridcache: wrote %d locators", len(batch))
 	return nil
 }