// that we have received the client's ACK, and at any rate we don't want to
	// allow a misbehaving client to soak up server connections indefinitely by
	// withholding an ACK, nor do we want to go through the complexity or overhead
	// of using low-level APIs to figure out when a TCP round-trip has completed.
	//
	// Instead, we declare that we are “reasonably certain” that we received the
	// ACK if maxRSTAvoidanceDelay has elapsed.