BUG/MINOR: quic: ensure cwnd limits are always enforced

BUG/MINOR: quic: ensure cwnd limits are always enforced

Congestion window is limit by a minimal and maximum values which can
never be exceeded. Min value is hardcoded to 2 datagrams as recommended
by the specification. Max value is specified via haproxy configuration.

These values must be respected each time the congestion window size is
adjusted. However, in some rare occasions, limit were not always
enforced. Fix this by implementing wrappers to set or increment the
congestion window. These functions ensure limits are always applied
after the operation.

Additionnally, wrappers also ensure that if window reached a new maximum
value, it is saved in <cwnd_last_max> field.

This should be backported up to 2.6, after a brief period of
observation.