message: Switch to F14FastMap for performance reasons
  Summary:
  F14FastMap is more efficient than std::unordered_map.
  TFunctionStatHandler::freeContext() is taking 0.5% of CPU in one particular
  service, this will improve that.
diff: #pragma once
  #include <fb303/BaseService.h>
  +#include <folly/container/F14Map.h>
  #include <folly/experimental/FunctionScheduler.h>
  #include <thrift/lib/cpp/TProcessor.h>
  @@ -223,14 +224,14 @@ class TFunctionStatHandler
  DynamicStrings dynamicStrings_; // holds the counters information
  ExportedHistogram dummyHist_;
  - std::unordered_map<std::string, ThriftFuncHistParams> histParamsMap_;
  + folly::F14FastMap<std::string, ThriftFuncHistParams> histParamsMap_;
  /**
  * Mapping from thrift functions to their respective
  * TStatsPerThread objects for a single thread
  */
  using TStatsAggregator =
  - std::unordered_map<std::string, std::shared_ptr<TStatsPerThread>>;
  + folly::F14FastMap<std::string, std::shared_ptr<TStatsPerThread>>;
  class Tag;
  folly::ThreadLocalPtr<TStatsAggregator, Tag> tlFunctionMap_;
  