message: Use folly::F14FastMap in QuicPriorityQueue
  Summary:
  There is a map in the QuicPriorityQueue that's used to look up a
  stream's level index. It doesn't need to be an ordered map.
diff: #pragma once
  +#include <folly/container/F14Map.h>
  #include <glog/logging.h>
  -#include <map>
  #include <set>
  #include <quic/codec/Types.h>
  @@ -49,7 +49,6 @@ struct PriorityQueue {
  bool incremental{false};
  };
  std::vector<Level> levels;
  - std::map<StreamId, size_t> writableStreams;
  PriorityQueue() {
  levels.resize(kDefaultPriorityLevels * 2);
  @@ -153,6 +152,7 @@ struct PriorityQueue {
  }
  private:
  + folly::F14FastMap<StreamId, size_t> writableStreams;
  using WSIterator = decltype(writableStreams)::iterator;
  WSIterator find(StreamId id) {
  