async populateCache() {
    if (global.kuzzle.config.services.storageEngine.generateMissingAliases) {
      await this.client.generateMissingAliases();
    }

    const schema = await this.client.getSchema();

    for (const [index, collections] of Object.entries(schema)) {
      this.cache.addIndex(index);

      for (const collection of collections) {
        this.cache.addCollection(index, collection);
      }
    }
  }
Populates the index cache with existing index/collection.
Also checks for duplicated index names.