Handle 413 Request Entity Too Large

[jakeichikawasalesforce]

Tornado has max_body_size as the largest response body that the client will accept, but the resulting code path does not respond with an HTTP error code of 413. By default, Tornado checks the content-length header in HTTP1Connection._read_body and raises an error if it exceeds the limit; the request handler code is never executed.

This PR overrides HTTP1Connection._read_body to allow requests that are too large to continue on to the handler. We then handle this scenario (Request Entity Too Large) in the base_handler with request_body_size_within_limit. This method is called at the beginning of each GET or POST request in the evaluation and query plane handlers.