BUG/MEDIUM: quic: Wrong K CUBIC calculation.

BUG/MEDIUM: quic: Wrong K CUBIC calculation.

The formula for K CUBIC calculation is as follows:

		K = cubic_root(W_max * (1 - beta_quic) / C).

Note that this does not match the comment. But the aim of this patch is to not
hide a bug inside another patch to update this K CUBIC calculation.

The unit of C is bytes/s^3 (or segments/s^3). And we want to store K as
milliseconds. So, the conversion inside the cubic_root() to convert seconds in
milliseconds is wrong. The unit used here is bytes/(ms/1000)^3 or
bytes*1000^3/ms^3. That said, it is preferable to compute K as seconds, then
convert to milliseconds as done by this patch.

Must be backported as far as 2.6.

(cherry picked from commit 406c63ba44666deb7f09a36748039aaafa8a7d5d)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>