Optimize skiplist random level generation logic (valkey-io#2631)

Each insertion of a skiplist node requires generating a random level
(via the `zslRandomLevel` function), and some commands (such as
`zunionstore`) call the `zslRandomLevel` function multiple times.
Therefore, optimizing `zslRandomLevel` can significantly improve the
performance of these commands.

The main optimization approach is as follows:

1. Original logic: Each iteration called the `random` function, with a
0.25 probability of continuing to call `random` again. In the worst-case
scenario, it required up to 32 calls (though the probability of this
occurring is extremely low).
2. Optimized logic: We only need to call the `genrand64_int64` function
once. Each iteration uses only 2 bits of randomness, effectively
achieving the equivalent of 32 iterations in the original algorithm.
3. Additionally, the introduction of `__builtin_clzll` significantly
reduces CPU usage, which compiles into a single, highly efficient CPU
instruction (e.g., LZCNT on x86, CLZ on ARM) on supported hardware
platforms
4. Although I've explained a lot, the actual code changes are quite
minimal, so just look at the code directly.

---------

Signed-off-by: chzhoo <czawyx@163.com>

Author: chzhoo

src/server.h

@@ -520,7 +520,6 @@ typedef enum {
 #define SUPERVISED_UPSTART 3
 
 #define ZSKIPLIST_MAXLEVEL 32 /* Should be enough for 2^64 elements */
-#define ZSKIPLIST_P 0.25      /* Skiplist P = 1/4 */
 #define ZSKIPLIST_MAX_SEARCH 10
 
 /* Append only defines */

src/t_zset.c

@@ -168,10 +168,14 @@ void zslFree(zskiplist *zsl) {
  * (both inclusive), with a powerlaw-alike distribution where higher
  * levels are less likely to be returned. */
 static int zslRandomLevel(void) {
-    static const int threshold = ZSKIPLIST_P * RAND_MAX;
-    int level = 1;
-    while (random() < threshold) level += 1;
-    return (level < ZSKIPLIST_MAXLEVEL) ? level : ZSKIPLIST_MAXLEVEL;
+    uint64_t rand = genrand64_int64();
+
+    /* The probability of gaining 2 additional leading zeros is 0.25.
+     * This matches the level calculation logic perfectly: each
+     * iteration has a 0.25 probability of increasing the level by 1.
+     * Note: __builtin_clzll has undefined behavior when the input is 0. */
+    int level = rand == 0 ? ZSKIPLIST_MAXLEVEL : (__builtin_clzll(rand) / 2 + 1);
+    return level;
 }
 
 /* Compares node and score/ele; defines zset ordering. Return value: