fix folly::NotificationQueue for move-only value types

Summary:
folly::NotificationQueue::tryConsume did not compile for
move-only types such as folly::Function. This diff fixes that.

Reviewed By: andriigrynenko

Differential Revision: D3711406

fbshipit-source-id: a52eba3eb31743165e4726f830f2a38d241f25a5

folly/io/async/NotificationQueue.h

@@ -422,9 +422,9 @@ class NotificationQueue {
       return false;
     }
 
-    auto data = std::move(queue_.front());
-    result = data.first;
-    RequestContext::setContext(data.second);
+    auto& data = queue_.front();
+    result = std::move(data.first);
+    RequestContext::setContext(std::move(data.second));
 
     queue_.pop_front();