Language Pack Upgrader: Allow to disable asynchronously translation updates.

* Don't touch VCS installs.
* Use the `async_update_translation` filter (which corresponds exactly to `auto_update_translation`) to entirely disable it, or based on the update offer. 

props nacin, ocean90.
fixes #28571.
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git-svn-id: http://core.svn.wordpress.org/trunk@29469 1a063a9b-81f0-0310-95a4-ce76da25c4cd