Use Baton (again) in EventBase::runInEventBaseThreadAndWait

Use Baton (again) in EventBase::runInEventBaseThreadAndWait

Summary:
`Baton` is more lightweight than a mutex+condition variable, and the code is much simpler. This was actually the original implementation, but the dependency had to be dropped because `Baton` was unsupported on some architectures. That is not a problem anymore.

Also reorganize the includes to follow the conventions.

Reviewed By: andriigrynenko, yfeldblum

Differential Revision: D5396631

fbshipit-source-id: d782cf271eb35723aaeb3c372e1c1dafeaaf0f0a