The music lovers of Boston gave John Williams about as fine a send-off as he could have wished at his swan song last Sunday night in Symphony Hall. There were five standing ovations, which drew five encores (including the rollicking theme from ''Flashdance''). After the fifth (''Cabaret''), the orchestra also stood and applauded the conductor. The musicians, with the audience, seemed to be telling Williams, ''Come back.'' But he waved his final goodbyes, which ended the evening, and probably an era.