use std::unordered_map in fiber-manager global maps

Summary:
The global maps are used only in the slow-path, so simplicity is better.

The simplicity also allows simpler access to the list of extant fiber-managers in the debugger, with needing the gdb scripts only to support `std::unordered_map` and not also `folly::F14NodeMap`.

Reviewed By: jaesoo-fb

Differential Revision: D45919505

fbshipit-source-id: fa354b60344809ff2a2e234945b3f19873b76ac4

third-party/folly/src/folly/fibers/FiberManagerMap-inl.h

@@ -17,6 +17,7 @@
 #pragma once
 
 #include <memory>
+#include <unordered_map>
 
 #include <folly/Function.h>
 #include <folly/ScopeGuard.h>
@@ -99,7 +100,7 @@ class GlobalCache {
   }
 
   std::mutex mutex_;
-  folly::F14NodeMap<Key<EventBaseT>, std::unique_ptr<FiberManager>> map_;
+  std::unordered_map<Key<EventBaseT>, std::unique_ptr<FiberManager>> map_;
 };
 
 constexpr size_t kEraseListMaxSize = 64;