Use easyjson to improve JSON marshal/unmarshal performance

[jimeh]

easyjson improves the performance of marshaling and unmarshaling JSON by
generating custom code for each of your structs. As the generated code is based
on your structs, it avoids using reflection.

• Add easyjson, generating code via go generate for all structs which are
  used for JSON requests/responses.
• Add CI step to ensure generated code is up to date, failing if it is not