Pass context by reference in the futures Core

Summary: [Folly] Pass context by reference in the futures `Core`, saving on code size and a small on runtime performance due to code size.

Reviewed By: Orvid

Differential Revision: D15073282

fbshipit-source-id: 7c55b9f6a55ee7ec22fde6f3ab9da232a8ee006b

folly/futures/detail/Core.h

@@ -303,7 +303,7 @@ class Core final {
   /// and might also synchronously execute that callback (e.g., if there is no
   /// executor or if the executor is inline).
   template <typename F>
-  void setCallback(F&& func, std::shared_ptr<folly::RequestContext> context) {
+  void setCallback(F&& func, std::shared_ptr<folly::RequestContext>&& context) {
     DCHECK(!hasCallback());
 
     // construct callback_ first; if that fails, context_ will not leak