Customizer: Introduce a processing state.

The processing state will prevent a save when it doesn't return 0.
Widget Customizer will use the state to prevent a save while a widget is updating, which is an asynchronous process.

props westonruter.
fixes #27390.

git-svn-id: https://develop.svn.wordpress.org/trunk@27540 602fd350-edb4-49c9-b593-d223f7449a82