evictEmpty() {
    while (this.indexes_.length > 1 && this.indexes_[0].isEmpty()) {
      const index = this.indexes_.shift();
      // In the case of this class, this.numEvicted_ represents the evicted
      // segments that were in indexes that were entirely evicted.
      this.numEvicted_ += index.getNumEvicted();
      index.release();
    }
  }
Evicts all old SegmentIndexes in this MetaSegmentIndex that are empty.