Use string not StringPiece for knobs
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 |??
```

Reviewed By: yangchi

Differential Revision: D24928201

fbshipit-source-id: 57f391516689b3e5a4fa941d22f464c9830ad660

facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst