message: Use folly::small_vector for acked ranges
  Summary: We expect this to be very small in most cases, as the ranges will normally get merged. Reducing a layer of indirection here helps on the ACK path.
diff: @@ -54,7 +54,10 @@ struct QuicStreamLike {
  // data being acked this would contain one internval from 0 -> the largest
  // offseet ACKed. This allows us to track which delivery callbacks can be
  // called.
  - IntervalSet<uint64_t> ackedIntervals;
  + template <class T>
  + using IntervalSetVec = SmallVec<T, 32, uint16_t>;
  + using AckedIntervals = IntervalSet<uint64_t, 1, IntervalSetVec>;
  + AckedIntervals ackedIntervals;
  // Stores a list of buffers which have been marked as loss by loss detector.
  // Each one represents one StreamFrame that was written.
  