MEDIUM: quic: define cubic-pacing congestion algorithm

MEDIUM: quic: define cubic-pacing congestion algorithm

Define a new QUIC congestion algorithm token 'cubic-pacing' for
quic-cc-algo bind keyword. This is identical to default cubic
implementation, except that pacing is used for STREAM frames emission.

This algorithm supports an extra argument to specify a burst size. This
is stored into a new bind_conf member named quic_pacing_burst which can
be reuse to initialize quic path.

Pacing support is still considered experimental. As such, 'cubic-pacing'
can only be used with expose-experimental-directives set.