Reference zset score in zskiplistNode from dict entries

This avoids the extra allocation of sizeof(double) for storing the score
of a zset entry in the hash table. Saves sizeof(double) + malloc
overhead = approx. 16 bytes per zset entry.

src/redis.h

@@ -675,7 +675,7 @@ void backgroundRewriteDoneHandler(int statloc);
 /* Sorted sets data type */
 zskiplist *zslCreate(void);
 void zslFree(zskiplist *zsl);
-void zslInsert(zskiplist *zsl, double score, robj *obj);
+zskiplistNode *zslInsert(zskiplist *zsl, double score, robj *obj);
 
 /* Core functions */
 void freeMemoryIfNeeded(void);