MINOR: quic-be: Store asap the DCID

MINOR: quic-be: Store asap the DCID

Store the peer connection ID (SCID) as the connection DCID as soon as an Initial
packet is received.
Stop comparing the packet to QUIC_PACKET_TYPE_0RTT is already match as
QUIC_PACKET_TYPE_INITIAL.
A QUIC server must not send too short datagram with ack-eliciting packets inside.
This cannot be done from quic_rx_pkt_parse() because one does not know if
there is ack-eliciting frame into the Initial packets. If the packet must be
dropped, this is after having parsed it!