MINOR: quic: rename min/max fields for congestion window algo

MINOR: quic: rename min/max fields for congestion window algo

There was some possible confusion between fields related to congestion
window size min and max limit which cannot be exceeded, and the maximum
value previously reached by the window.

Fix this by adopting a new naming scheme. Enforced limit are now renamed
<limit_max>/<limit_min>, while the previously reached max value is
renamed <cwnd_last_max>.

This should be backported up to 3.1.

(cherry picked from commit 2eb1b0cd96f663b9260ee48921612566417b3b8d)
Signed-off-by: Willy Tarreau <w@1wt.eu>