change the polling timer to 30ms for both read and write queue

src/dnode/src/dnodeRead.c

@@ -225,7 +225,7 @@ static void dnodeHandleIdleReadWorker(SReadWorker *pWorker) {
     dTrace("read worker:%d is released, total:%d", pWorker->workerId, readPool.num);
     pthread_exit(NULL);
   } else {
-    usleep(100);
+    usleep(30000);
     sched_yield();
   }
 }

src/dnode/src/dnodeWrite.c

@@ -227,7 +227,7 @@ static void dnodeHandleIdleWorker(SWriteWorker *pWorker) {
   int32_t num = taosGetQueueNumber(pWorker->qset);
 
   if (num > 0) {
-     usleep(1000);
+     usleep(30000);
      sched_yield(); 
   } else {
      taosFreeQall(pWorker->qall);