public Builder executor(ListeningExecutorService executor) {
      this.executor = checkNotNull(executor);
      return this;
    }
Configures repository to use specified executor for all operations.
@param executor executor that will be used by repositories.
@return {@code this}
@see Executors
@see MoreExecutors#listeningDecorator(java.util.concurrent.ExecutorService)
mongo/src/org/immutables/mongo/repository/RepositorySetup.java