redis scan command support

[lukemakeit]

Problem

redis scan command not support.

Solution

Note: server_idx is saved by the lowest 12 bits of cursor, so the number of servers of the twemproxy must be less than 4096.

Result

1.1.1.1:60100> scan 0
1) "1"
2) 1) "d"
   2) "b"
   3) "c"
   4) "a"
1.1.1.1:60100> scan 1
1) "0"
2) 1) "a"
   2) "b"

[lukemakeit]

Hi @wy-ei , I have re-committed according to your suggestion, thanks.

[wy-ei]

@lukexwang

I have see the change you made. it's better. And I think the code can be optimized again. I have leave some more suggestions.

[lukemakeit]

Hi @wy-ei , thank you for your suggestion. I have re-committed.

[lukemakeit]

Hello @wy-ei , 空了辛苦看下哈 ^_^

[yuerugou54]

马晓栋已经收到您的来件，谢谢!