MINOR: quic: adapt credit based pacing to BBR

MINOR: quic: adapt credit based pacing to BBR

Credit based pacing has been further refined to be able to calculate
dynamically burst size based on congestion parameter. However, BBR
algorithm already provides pacing rate and burst size (labelled as
send_quantum) for 1ms of emission.

Adapt quic_pacing_reload() to use BBR values to compute pacing credit.
This is done via pacing_burst callback which is now only defined for
BBR. For other algorithms, determine the burst size over 1ms with the
congestion window size and RTT.

This should be backported up to 3.1.