修改session生成逻辑。

@@ -130,8 +130,11 @@ static dispatch_queue_t persisitingQueue;
 
 - (NSString *)session
 {
-    NSString *rawSession = [[OpenUDID value] stringByAppendingString:[self timestamp]];
-    return [rawSession md5];
+    if (!_session || [_session isEqualToString:@""]) {
+        _session = [[OpenUDID value] stringByAppendingString:[self timestamp]];
+    }
+    
+    return [_session md5];
 }
 
 #pragma mark - Functions