FlatCombining: Use SaturatingSemaphore instead of multi-poster and non-blocking Baton
FlatCombining: Use SaturatingSemaphore instead of multi-poster and non-blocking Baton

Summary: Use SaturatingSemaphore instead of multi-poster and non-blocking Baton, in preparation for restoring Baton to being single-poster and with blocking wait().

Reviewed By: davidtgoldblatt

Differential Revision: D6510619

fbshipit-source-id: eed87beabc1b698080b8e41c746d570d30804ce7

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