Explicitly enforce that RequestToken is trivially-destructible

Summary:
[Folly] Explicitly enforce that `RequestToken` is trivially-destructible to permit its use as a Meyers Singleton without requiring the use of Leaky Meyers Singleton to avoid destruction-order problems.

(Note: this ignores all push blocking failures!)

Differential Revision: D21533605

fbshipit-source-id: fe334fd5257b1d0a07efd849c4532b28801e54c4

folly/io/async/Request.h

@@ -55,6 +55,9 @@ class RequestToken {
 
   uint32_t token_;
 };
+static_assert(
+    std::is_trivially_destructible<RequestToken>::value,
+    "must be trivially destructible");
 
 } // namespace folly