Use relaxed CAS in Baton wait path
Use relaxed CAS in Baton wait path

Summary:
[Folly] Use relaxed CAS in `Baton` wait path.

As is done in `SaturatingSemaphore`. There are surrounding operations on `state_` with stronger memory orders.

Reviewed By: nbronson

Differential Revision: D6726704

fbshipit-source-id: 8032db4b98a09455c84c730e2e99515d1a9442da

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