message: Move assignment of LongHeader
  Summary:
  In AckHandler we try to move OutstandingPacket. But LongHeaer has been
  copied. Add a move assignment for LongHeader to avoid the copy.
diff: @@ -658,6 +658,11 @@ struct LongHeader {
  const std::string& token = std::string(),
  folly::Optional<ConnectionId> originalDstConnId = folly::none);
  + LongHeader(const LongHeader& other) = default;
  + LongHeader(LongHeader&& other) = default;
  + LongHeader& operator=(const LongHeader& other) = default;
  + LongHeader& operator=(LongHeader&& other) = default;
  +
  Types getHeaderType() const noexcept;
  const ConnectionId& getSourceConnId() const;
  const ConnectionId& getDestinationConnId() const;
  