Add logged request headers to MDC

...so that we can trace every log line of a request with the `X-Socrata-RequestId`, and any other headers we want, like the 4x4 / Resource.
Should clear it after we log the response headers?  I can see, for example, us wanting the request id to be logged in each line from the MDC, and if we clear it here then it won't be on the line that logs the X-SODA2-Rollup header so we won't be able to find that easily for a request.