任务触发组件优化，常规1s预加载一次，预加载轮空时主动休眠4s，降低加载频率以及对DB压力；

xxl-job-admin/src/main/java/com/xxl/job/admin/core/thread/JobScheduleHelper.java

@@ -194,8 +194,8 @@ public class JobScheduleHelper {
                     // Wait seconds, align second
                     if (cost < 1000) {  // scan-overtime, not wait
                         try {
-                            // pre-read success, exist job in 5s, wait 1s;
+                            // pre-read success, exist job in pre-read period, wait 1s;
-                            // pre-read fail, no exist job in 5s, wait 1s
+                            // pre-read fail, no exist job in pre-read period, wait 4s
                             TimeUnit.MILLISECONDS.sleep((preReadSuc?1000:4000) - System.currentTimeMillis()%1000);
                         } catch (InterruptedException e) {
                             if (!scheduleThreadToStop) {