core: hide registerHandler (#964)

* core: hide registerHandler

Remove RegisterHandler and just make it implicit when we look at the
handler compilation step.

* Rename GetHandler to just Handler

Update callers and make auto check Hander in OnStartup.

* Up test coverage in erratic

* up test coverage

middleware/erratic/setup.go

@@ -28,9 +28,6 @@ func setupErratic(c *caddy.Controller) error {
 		return e
 	})
 
-	// Also register erratic for use in autopath.
-	dnsserver.GetConfig(c).RegisterHandler(e)
-
 	return nil
 }