MINOR: quic: compare coalesced packets by DCID

MINOR: quic: compare coalesced packets by DCID

If an UDP datagram contains multiple QUIC packets, they must all use the
same DCID. The datagram context is used partly for this.

To ensure this, a comparison was made on the dcid_node of DCID tree. As
this is a comparison based on pointer address, it can be faulty when
nodes are removed/readded on the same pointer address.

Replace this comparison by a proper comparison on the DCID data itself.
To this end, the dgram_ctx structure contains now a quic_cid member.