message: Use F14FastMap in ExportedStatMap
  Summary: `StringKeyedUnorderedMap` is a legacy wrapper from when we didn't have F14 with heterogeneous lookup, and it's not necessary anymore. It uses `F14NodeMap`, which requires an allocation on every insertion. Instead, switch to `F14FastMap`.
diff: #include <fb303/ExportType.h>
  #include <fb303/MutexWrapper.h>
  #include <fb303/Timeseries.h>
  -#include <folly/experimental/StringKeyedUnorderedMap.h>
  +#include <folly/container/F14Map.h>
  namespace facebook {
  namespace fb303 {
  @@ -39,7 +39,7 @@ class ExportedStatMap {
  using SyncStat = folly::Synchronized<ExportedStat, MutexWrapper>;
  using StatPtr = std::shared_ptr<SyncStat>;
  using LockedStatPtr = SyncStat::LockedPtr;
  - using StatMap = folly::StringKeyedUnorderedMap<StatPtr>;
  + using StatMap = folly::F14FastMap<std::string, StatPtr>;
  /*
  * Creates an ExportedStatMap and hooks it up to the given DynamicCounters
  