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.
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git-svn-id: http://core.svn.wordpress.org/trunk@27383 1a063a9b-81f0-0310-95a4-ce76da25c4cd