code: function apply_write (key, value, err) {
      var _reading = reading[key]
      reading[key] = null
      while(_reading && _reading.length)
        _reading.shift()(err, value)
      _write()
    }
docs: this writes only once at a time. at least, we want it to write at most once per file. or maybe if it's written recently then wait. anyway, this is good enough for now.