Reject a request with huge payload which exceeds the server's size limit

[yhirose]

#142 (comment)

Another thing I noticed in our stress testing today, we have to reject streams if the content length exceeds a maximum size specified in the server instance. Otherwise, bad clients can create 40gb requests - effectively killing our process.

• Server::set_payload_max_length(uint64_t length) to set limit size
• 413 Payload Too Large will return when the body size exceeds the limit