Fix synchronization/correctness issues

[franklinhu]

Using a singleton FinatraResponse object has a variety of issues:

• Doesn't work for multithreaded applications
• Using a single resp allows data to persist across requests if those fields aren't overridden such as:
  1. first request hits endpoint rendered with a JSON body
  2. second request hits endpoint without a body, but will inherit the previous request's body
• This is fixed by not using an singleton and instantiating a new FinatraResponse for each request