Conifex Timber Inc. said it will temporarily slow production at its sawmill in Mackenzie, British Columbia, due to an “unprecedented collapse in lumber prices.” It will curtail production for two weeks starting Aug. 23.  North American lumber companies may have added too much production, too quickly, as demand cools and prices for the construction material plunge. After more than quadrupling in 12 months to record highs, lumber has sunk about 70% since May. The crunch is particularly acute in Western Canada because of higher costs.  Producers were swimming in cash earlier this year after rock-bottom ...