[dataset] Thread device should send /c/ps when attaching (#2000)

On attach, a device clears its network pending dataset.  If the device
has a newer local pending dataset, it should generate and send a /c/ps
message to the Leader.

src/core/meshcop/dataset_manager.cpp

@@ -1043,6 +1043,7 @@ void PendingDatasetBase::ClearNetwork(void)
 {
     mNetwork.Clear();
     mDelayTimer.Stop();
+    DatasetManager::HandleNetworkUpdate();
 }
 
 void PendingDatasetBase::HandleDetach(void)

src/core/meshcop/dataset_manager.hpp

@@ -440,7 +440,6 @@ protected:
     static void HandleDelayTimer(Timer &aTimer);
     void HandleDelayTimer(void);
     void StartDelayTimer(void);
-    void HandleNetworkUpdate(void);
 
     TimerMilli mDelayTimer;