MC importer doesn't work when leader controller is restarted

[luolanzone]

Describe the bug

The importer of member cluster in a ClusterSet will stop working when the leader controller is restarted. Importer in MC is watching the events from leader cluster after a ClusterSet is initialized, when the leader controller is restarted, leader election can complete successfully but it doesn't start a new importer after election is done. This means member cluster can't import any new resource anymore if a leader controller is crashed and recovered.

To Reproduce
Set up a Antrea multi-cluster ClusterSet, trying so basic Service export/import, then restart the leader controller.

Expected
The importer should be started again after a new leader election is finished after the leader controller is restarted.