RequestContext: replace F14FastMap by std::unordred_map

Summary: Production data shows that it performs significantly worse under common RequestContext load (few elements, clang lto compiled).

Reviewed By: djwatson

Differential Revision: D9182929

fbshipit-source-id: 0c638268d9b6cd2451b48ccc4e228faf90f0e18e

folly/io/async/Request.h

@@ -16,12 +16,12 @@
 
 #pragma once
 
-#include <folly/container/F14Map.h>
-#include <folly/sorted_vector_types.h>
 #include <memory>
 #include <string>
+#include <unordered_map>
 
 #include <folly/Synchronized.h>
+#include <folly/sorted_vector_types.h>
 
 namespace folly {
 
@@ -169,7 +169,7 @@ class RequestContext {
 
   struct State {
     // This must be optimized for lookup, its hot path is getContextData
-    F14FastMap<std::string, RequestData::SharedPtr> requestData_;
+    std::unordered_map<std::string, RequestData::SharedPtr> requestData_;
     // This must be optimized for iteration, its hot path is setContext
     // We also use the fact that it's ordered to efficiently compute
     // the difference with previous context