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/setup.go

@@ -38,6 +38,10 @@ func setup(c *caddy.Controller) error {
 			return nil
 		})
 	}
+	for _, n := range zones.Names {
+		z := zones.Z[n]
+		c.OnShutdown(z.OnShutdown)
+	}
 
 	dnsserver.GetConfig(c).AddPlugin(func(next plugin.Handler) plugin.Handler {
 		return File{Next: next, Zones: zones}