QUIC: refactored packet processing. - split ngx_quic_process_packet() in two functions with the second one called ngx_quic_process_payload() in charge of decrypring and handling the payload - renamed ngx_quic_payload_handler() to ngx_quic_handle_frames() - moved error cleanup from ngx_quic_input() to ngx_quic_process_payload() - moved handling closed connection from ngx_quic_handle_frames() to ngx_quic_process_payload() - minor fixes