Avoid calling _advanceReadiness() too many times in dropInstance()

By reading the code, I come to conclusion that _advanceReadiness() should be called exactly as many times as _deferReadiness(), per dbContext.
But in dropInstance() it is called for each forage of the dbContext.

src/drivers/indexeddb.js

@@ -1007,6 +1007,7 @@ function dropInstance(options, callback) {
                         for (var i = 0; i < forages.length; i++) {
                             const forage = forages[i];
                             _advanceReadiness(forage._dbInfo);
+                            break;
                         }
                     })
                     .catch(err => {