Skip shared_ptr in futures waitImpl

Summary:
wait() is guaranteed to succeed and not throw for either Baton type, so
using a reference here is safe. When the future is completed on another thread,
the shared_ptr causes cache contention.

```
Thread 1            Thread 2
make_shared/inc
inc
...
                    dec ref
dec ref/free
```

Reviewed By: yfeldblum

Differential Revision: D17077672

fbshipit-source-id: a6010ed159353decefbd5c51f4347370b16f7f60

folly/futures/Future-inl.h

@@ -2125,14 +2125,14 @@ void waitImpl(FutureType& f) {
 
   Promise<T> promise;
   auto ret = convertFuture(promise.getSemiFuture(), f);
-  auto baton = std::make_shared<FutureBatonType>();
-  f.setCallback_([baton, promise = std::move(promise)](
+  FutureBatonType baton;
+  f.setCallback_([&baton, promise = std::move(promise)](
                      Executor::KeepAlive<>&&, Try<T>&& t) mutable {
     promise.setTry(std::move(t));
-    baton->post();
+    baton.post();
   });
   f = std::move(ret);
-  baton->wait();
+  baton.wait();
   assert(f.isReady());
 }