QUIC: simplified quic connection dispatching.

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.