Replace std::unique_lock with std::lock_guard.

Summary:
std::lock_guard has a better performance than std::unique_lock in scoped-lock.
Closes https://github.com/facebook/folly/pull/840

Reviewed By: djwatson

Differential Revision: D7888672

Pulled By: yfeldblum

fbshipit-source-id: 391082aa79ada67a3906e828d68f8cc2a29bf22c

folly/synchronization/ParkingLot.h

@@ -59,7 +59,7 @@ struct WaitNodeBase {
   }
 
   void wake() {
-    std::unique_lock<std::mutex> nodeLock(mutex_);
+    std::lock_guard<std::mutex> nodeLock(mutex_);
     signaled_ = true;
     cond_.notify_one();
   }