iterator = function (callback) {
    return function (err) {
      queue.shift()
      callback(err)

      const next = queue[0]
      if (next) {
        processor(next.fs, next.data, iterator(next.callback))
      }
    }
  }
Takes in a processor function, and returns a writer function.

@param {Function} processor

@return {Function} queuedWriter