Starbucks baristas’ fast-growing unfair labor practice strike is expected to hit historic levels Tuesday as hundreds of stores nationwide prepare to join walkouts at the coffee giant. The Starbucks Workers United strike, prompted by failed union contract negotiations, began Friday and is expected to continue to escalate during the holidays – potentially affecting more than 300 stores nationwide, according to Workers United.Baristas in Boston, Philadelphia, Portland and Tucson walked out Monday and joined their coworkers…