Return 431 status when headers are too large

When a request's headers exceed BufferSize we currently handle it as a DRC.Wait error and close the connection. I believe the more proper way to handle it is to issue a HTTP 431. This will give the client more information on how to correct the request.

[431 Request Header Fields Too Large](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/431)

The server is unwilling to process the request because its header fields are too large. The request may be resubmitted after reducing the size of the request header fields.