code: function ltrim(key, start, stop, req) {
    var val = this.getKey(key, req);
    if(val === undefined) return null;
    val.ltrim(start, stop);
    if(val.llen() === 0) {
      this.delKey(key, req);
    }
    return OK;
  }
docs: Trim an existing list so that it will contain only the specified
  range of elements.