Common tree insert function for QUIC and UDP connections. Previously, ngx_udp_rbtree_insert_value() was used for plain UDP and ngx_quic_rbtree_insert_value() was used for QUIC. Because of this it was impossible to initialize connection tree in ngx_create_listening() since this function is not aware what kind of listening it creates. Now ngx_udp_rbtree_insert_value() is used for both QUIC and UDP. To make is possible, a generic key field is added to ngx_udp_connection_t. It keeps client address for UDP and connection ID for QUIC.