cubic: limit cwnd growth based on max bytes-in-flight
cubic: limit cwnd growth based on max bytes-in-flight

When cwnd is updated in processing acks, limit its high
bound to 2 x max_bytes_in_flight. max_bytes_in_flight is
max value of bytes_in_flight during the connection lifetime.

This makes cwnd not to exceed too much, roughly bounded
by the receiver's flow control window.

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