Use compact_once_flag for DelayedInit

Summary: Use the 1 byte version of `once_flag` such that there is no more memory overhead than `Optional`.

Reviewed By: andriigrynenko

Differential Revision: D25909346

fbshipit-source-id: 1bb2f41200917ba58b20815bfafe3314137e9680

folly/synchronization/DelayedInit.h

@@ -153,7 +153,7 @@ struct DelayedInit {
     union {
       std::remove_const_t<T> value;
     };
-    once_flag init;
+    compact_once_flag init;
 
     StorageTriviallyDestructible() {}
   };
@@ -162,7 +162,7 @@ struct DelayedInit {
     union {
       std::remove_const_t<T> value;
     };
-    once_flag init;
+    compact_once_flag init;
 
     StorageNonTriviallyDestructible() {}
     ~StorageNonTriviallyDestructible() {