Computes the difference between the first and all successive input sorted sets
and stores the result in destination. The total number of input keys is
specified by numkeys.
Keys that do not exist are considered to be empty sets.
If destination already exists, it is overwritten (when the difference is nonempty) or deleted (when the difference is empty).
@examples
ZADD zset1 1 "one"
ZADD zset1 2 "two"
ZADD zset1 3 "three"
ZADD zset2 1 "one"
ZADD zset2 2 "two"
ZDIFFSTORE out 2 zset1 zset2
ZRANGE out 0 -1 WITHSCORES