add sleeper, to reduce cpu usage.

lichao

2021-05-20 6b96ee857e366af317ab9dc1b6b4f9c22576e83b

 src/exported_symbols

@@ -5,6 +5,7 @@
   BHUnregister;
   BHRegisterTopics;
   BHQueryTopicAddress;
   BHQueryProcs;
   BHSubscribeTopics;
   BHStartWorker;
   BHHeartbeatEasy;