docs: protected boolean startFlush() {
        if (!scheduledKeys.isEmpty()) {
           tryRegularInvalidations(true);
           return true;
        } else {
           // when there are some invalidations, removed invalidations are triggered automatically
           // but here we have to start it manually
           if (!removedKeys.isEmpty()) {
              tryRemovedInvalidations();
              return true;
           } else {
              return false;
           }
        }
     }