Send hearbeat during warm reboot freese (sonic-net#2923)

*Orchagent send heartbeat during warm-reboot to prevent Orchagent stuck alert.

Author: liuh-80

orchagent/orchdaemon.cpp

@@ -865,7 +865,7 @@ void OrchDaemon::start()
                     flush();
 
                     SWSS_LOG_WARN("Orchagent is frozen for warm restart!");
-                    sleep(UINT_MAX);
+                    freezeAndHeartBeat(UINT_MAX);
                 }
             }
         }
@@ -1034,6 +1034,19 @@ void OrchDaemon::heartBeat(std::chrono::time_point<std::chrono::high_resolution_
     }
 }
 
+void OrchDaemon::freezeAndHeartBeat(unsigned int duration)
+{
+    while (duration > 0)
+    {
+        // Send heartbeat message to prevent Orchagent stuck alert.
+        auto tend = std::chrono::high_resolution_clock::now();
+        heartBeat(tend);
+
+        duration--;
+        sleep(1);
+    }
+}
+
 FabricOrchDaemon::FabricOrchDaemon(DBConnector *applDb, DBConnector *configDb, DBConnector *stateDb, DBConnector *chassisAppDb, ZmqServer *zmqServer) :
     OrchDaemon(applDb, configDb, stateDb, chassisAppDb, zmqServer),
     m_applDb(applDb),

orchagent/orchdaemon.h

@@ -101,6 +101,8 @@ class OrchDaemon
     void flush();
 
     void heartBeat(std::chrono::time_point<std::chrono::high_resolution_clock> tcurrent);
+
+    void freezeAndHeartBeat(unsigned int duration);
 };
 
 class FabricOrchDaemon : public OrchDaemon