message: Use string not StringPiece for knobs
  Summary:
  quicConfig is passed by value(https://fburl.com/codesearch/rydlyc2x), StringPiece doesn't do copy, resulting in junk values.
  ```
  E1112 10:45:19.545003  1377 TransportKnobs.cpp:44] fail to parse knobs: json parse error on line 0 near |??
  ```
diff: @@ -102,11 +102,11 @@ struct D6DConfig {
  ProbeSizeRaiserType raiserType{ProbeSizeRaiserType::ConstantStep};
  };
  -// JSON-serialized transoprt knobs
  +// JSON-serialized transport knobs
  struct SerializedKnob {
  uint64_t space;
  uint64_t id;
  - folly::StringPiece blob;
  + std::string blob;
  };
  struct TransportSettings {
  