allowing sort in read-only-slave mode and only deny sort Command with st...

Allowing sort command in read-only-slave

1] removing "w" attribute from sort Command
2] and prohibit writing with store option in sortCommand when it is read only slave.

but. I might think this is not a good approach. because it is weak that someone add new option to new option.

so. how about add some other command only to store it's result to list.(ex. sortAndStore)

```
127.0.0.1:6380> sort key desc
1) "20"
2) "10"
3) "5"
4) "4"
5) "3"
6) "2"
7) "1"
127.0.0.1:6380> sort key desc store k
(error) READONLY You can't write against a read only slave.
127.0.0.1:6380> 
```