QUIC: packet processing refactoring. All packet header parsing is now performed by ngx_quic_parse_packet() function, located in the ngx_quic_transport.c file. The packet processing is centralized in the ngx_quic_process_packet() function which decides if the packet should be accepted, ignored or connection should be closed, depending on the connection state. As a result of refactoring, behavior has changed in some places: - minimal size of Initial packet is now always tested - connection IDs are always tested in existing connections - old keys are discarded on encryption level switch