xscan supported

hscan/sscan/zscan implement. 

there is a trick on handler hscan/sscan/zscan response, which has nested multi-bulk 

these command replay a nested multi-bulk with a number and a multi bulk like this:

    - mulit-bulk
     - cursor
     - mulit-bulk
        - val1
        - val2
        - val3

in this case, there is only one sub-multi-bulk,
and it's the last element of parent, we can handle it like tail-recursive.