code: function lset(key, index, value, req) {
    var val = this.getKey(key, req);
    if(val === undefined) return null;
    val = val.lset(index, value);
    return val !== null ? OK : null;
  }
docs: TODO: blpop TODO: brpop TODO: brpoplpush 
  Sets the list element at index to value.