Returns the score of member in the sorted set at key.
If member does not exist in the sorted set, or key does not exist, nil is
returned.
*Return value
Bulk string reply: the score of member (a double precision floating point number),
represented as string.
*Examples
redis>
ZADD myzset 1 "one"
(integer) 1redis> ZSCORE myzset "one"
"1"