MINOR: quic: rename pacing_rate cb to pacing_inter

MINOR: quic: rename pacing_rate cb to pacing_inter

Rename one of the congestion algorithms pacing callback from pacing_rate
to pacing_inter. This better reflects that this function returns a delay
(in nanoseconds) which should be applied between each packet emission to
fill the congestion window with a perfectly smoothed emission.

This should be backported up to 3.1.