MINOR: quic: quic_cc modifications to support BBR

MINOR: quic: quic_cc modifications to support BBR

Add several callbacks to quic_cc_algo struct which are only called by BBR.
->get_drs() may be used to retrieve the delivery rate sampling information
from an congestion algorithm struct (quic_cc).
->on_transmit() must be called before sending any packet a QUIC sender.
->on_ack_rcvd() must be called after having received an ACK.
->on_pkt_lost() must be called after having detected a packet loss.
->congestion_event() must be called after any congestion event detection
Modify quic_cc.c to call ->event only if defined. This is not the case
for BBR.