feat: refactor FPUser getKey function

Author: jianggang

src/main/java/com/featureprobe/sdk/server/FPUser.java

@@ -60,6 +60,9 @@ public boolean containAttr(String name) {
      * @return key
      */
     public String getKey() {
+        if (key == null) {
+            this.key = String.valueOf(System.nanoTime());
+        }
         return key;
     }
 

src/main/java/com/featureprobe/sdk/server/model/Split.java

@@ -35,8 +35,7 @@ public Split(List<List<List<Integer>>> distribution) {
     }
 
     public HitResult findIndex(FPUser user, String toggleKey) {
-        String hashKey = user.getKey() != null ? user.getKey() : String.valueOf(System.nanoTime());
-        user.stableRollout(hashKey);
+        String hashKey = user.getKey();
         if (StringUtils.isNotBlank(bucketBy)) {
             if (user.containAttr(bucketBy)) {
                 hashKey = user.getAttr(bucketBy);