The Midcoast Symphony Orchestra presented four ambitious and varied programs this season, and for the last of them, Rohan Smith, the orchestra’s music director, pulled out all the stops: Joining forces with the Vox Nova Chamber Choir, the Oratorio Chorale and a balanced quartet of vocal soloists, Smith assembled an ensemble of about 160 players and singers for performances of Giuseppe Verdi’s spectacularly operatic Messa da Requiem.