(PITTSFIELD, Mass.) – Boston-based folk-rock collective Session Americana kicks off the 2015 Mr. Finn’s Cabaret season at Barrington Stage Company (BSC) on Sunday, May 17, and Monday, May 18, at 8pm. A kind of all-star supergroup of Boston-based roots-music talent, the core of the band includes singer-songwriter Ry Cavanaugh, drummer Billy Beard (Patty Griffin), multi-instrumentalist…