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