MINOR: quic: More precise window update calculation

MINOR: quic: More precise window update calculation

When in congestion avoidance state and when acknowledging an <acked> number bytes
we must increase the congestion window by at most one datagram (<path->mtu>)
by congestion window. So thanks to this patch we apply a ratio to the current
number of acked bytes : <acked> * <path->mtu> / <cwnd>.
So, when <cwnd> bytes are acked we precisely increment <cwnd> by <path->mtu>.
Furthermore we take into an account the number of remaining acknowledged bytes
each time we increment the window by <acked> storing their values in the algorithm
struct state (->remain_acked) so that it might be take into an account at the
next ACK event.