MEDIUM: quic: implement credit based pacing

MEDIUM: quic: implement credit based pacing

Implement a new method for QUIC pacing emission based on credit. This
represents the number of packets which can be emitted in a single burst.
After emission, decrement from the credit the number of emitted packets.
Several emission can be conducted in the same sequence until the credit
is completely decremented.

When a new emission sequence is initiated (i.e. under a new QMUX tasklet
invokation), credit is refilled according to the delay which occured
between the last and current emission context.

This new mechanism main advantage is that it allows to conduct several
emission in the same task context without having to wait between each
invokation. Wait is only forced if pacing is expired, which is now
equivalent to having a null credit.

Furthermore, if delay between two emissions sequence would have been
smaller than expected, credit is only partially refilled. This allows to
restart emission without having to wait for the whole credit to be
available.

On the implementation side, a new field <credit> is avaiable in
quic_pacer structure. It is automatically decremented on
quic_pacing_sent_done() invokation. Also, a new function
quic_pacing_reload() must be used by QUIC MUX when a new emission
sequence is initiated to refill credit. <next> field from quic_pacer has
been removed.

For the moment, credit is based on the burst configured via quic-cc-algo
keyword, or directly reported by BBR.

This should be backported up to 3.1.