Merge folly/futures/helpers.h into folly/futures/Future.h

Summary: [Folly] Merge `folly/futures/helpers.h` into `folly/futures/Future.h` to avoid the cyclic include trap.

Differential Revision: D15600550

fbshipit-source-id: a0d14c0092bb433f7af797201d9fafebab370502

folly/futures/Future-inl.h

@@ -16,6 +16,7 @@
 #pragma once
 
 #include <algorithm>
+#include <atomic>
 #include <cassert>
 #include <chrono>
 #include <thread>