Sort keys by memory use/ttl

[bzatrok]

The memory footprint of a key is visibly only if the key is opened.
It would be useful to be able to sort keys by size largest to smallest, to see if memory is used inefficiently by specific keys.

It would also be useful to be able to sort keys by TTL to see which keys are about to expire.

[qishibo]

1. you need some rdbtools such as rdb or https://github.com/xueqiu/rdr to analysis key space[they usually use a rdb file for that], or it's very difficult for a redis client to implement that.
2. it's easy to implement, but a little performance cost, because it need to exec ttl for every key in the list, if you key list is very large, this is unfriendly