json iterator

Add import "github.com/json-iterator/go" and replace json.Marshal with jsoniter.Marshal. Then the code should behave exactly the same, just much faster. Unlike easyjson or other json libaries, jsoniter does not rely on static code generation. Unmarshal, NewEncoder, NewDecoder they all works. Existing types implemented Marshaler or Unmarshaler interface will also work. Map with non-string key also work. Yes, everything just works. The default performance is already several times faster than the standard library. Use jsoniter.ConfigFastest, this will marshal the float with 6 digits precision (lossy), which is significantly faster.