跳跃表

跳跃表的定义在server.h中

zskiplistNode代表跳表中的各个节点，zskiplist代表跳表。

zskiplist表示跳表，有头结点、尾节点、总长度和层级。

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typedef struct zskiplistNode {
    sds ele;
    double score;
    struct zskiplistNode *backward;
    struct zskiplistLevel {
        struct zskiplistNode *forward;
        unsigned long span;
    } level[];
} zskiplistNode;

typedef struct zskiplist {
    struct zskiplistNode *header, *tail;
    unsigned long length;
    int level;
} zskiplist;