Return 431 status when headers are too large

[bpbecker]

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

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.

[bpbecker]

This will require a change to Conga