BUG/MINOR: quic: Lack of precision when computing K (cubic only cc)

BUG/MINOR: quic: Lack of precision when computing K (cubic only cc)

K cubic variable is stored in ms. But it was a formula with the second as unit
for the window difference parameter which was used to compute K without
considering the loss of information. Then the result was converted in ms (K *= 1000).
This leaded to a lack of precision and multiples of 1000 as values.

To fix this, use the same formula but with the window difference in ms as parameter
passed to the cubic function and remove the conversion.

Must be backported as far as 2.6.