plugin/file: shutdown reload goroutine (#1571)

* plugin/file: shutdown reload goroutine

Shutdown the z.Reload() routine (if started in the first place) on
shutdow and reload.

Fixes #1508

* Must be put in c.OnShutdown()

* up test coverage

plugin/file/shutdown.go

@@ -0,0 +1,9 @@
+package file
+
+// OnShutdown shuts down any running go-routines for this zone.
+func (z *Zone) OnShutdown() error {
+	if !z.NoReload {
+		z.reloadShutdown <- true
+	}
+	return nil
+}