QUIC: simplified quic connection dispatching. Currently listener contains rbtree with multiple nodes for single QUIC connection: each corresponding to specific server id. Each udp node points to same ngx_connection_t, which points to QUIC connection via c->udp field. Thus when an event handler is called, it only gets ngx_connection_t with c->udp pointing to QUIC connection. This makes it hard to obtain actual node which was used to dispatch packet (it requires to repeat DCID lookup). Additionally, ngx_quic_connection_t->udp field is only needed to keep a pointer in c->udp. The node is not added into the tree and does not carry useful information.