There are worse inconveniences than having to wear a face mask to the opera. But there’s one consequence that hadn’t really struck home until an hour into Rimsky-Korsakov’sIvan the Terrible. The citizens of Pskov are massing in the streets. The Tsar’s army is approaching, and Rimsky is building one of those surging Russian crowd scenes: bass-heavy chorus blazing away while ominous bell sounds — basses, horns and rasping gong — shake the orchestra to its bones.