lease,integration: add checkpoint scheduling after leader change

Current checkpointing mechanism is buggy. New checkpoints for any lease
are scheduled only until the first leader change. Added fix for that
and a test that will check it.

server/lease/lessor.go

@@ -446,6 +446,7 @@ func (le *lessor) Promote(extend time.Duration) {
 		l.refresh(extend)
 		item := &LeaseWithTime{id: l.ID, time: l.expiry}
 		le.leaseExpiredNotifier.RegisterOrUpdate(item)
+		le.scheduleCheckpointIfNeeded(l)
 	}
 
 	if len(le.leaseMap) < leaseRevokeRate {

server/lease/lessor_test.go

@@ -531,6 +531,7 @@ func TestLessorCheckpointScheduling(t *testing.T) {
 	defer be.Close()
 
 	le := newLessor(lg, be, LessorConfig{MinLeaseTTL: minLeaseTTL, CheckpointInterval: 1 * time.Second})
+	defer le.Stop()
 	le.minLeaseTTL = 1
 	checkpointedC := make(chan struct{})
 	le.SetCheckpointer(func(ctx context.Context, lc *pb.LeaseCheckpointRequest) {
@@ -543,13 +544,11 @@ func TestLessorCheckpointScheduling(t *testing.T) {
 			t.Errorf("expected checkpoint to be called with Remaining_TTL=%d but got %d", 1, c.Remaining_TTL)
 		}
 	})
-	defer le.Stop()
-	le.Promote(0)
-
 	_, err := le.Grant(1, 2)
 	if err != nil {
 		t.Fatal(err)
 	}
+	le.Promote(0)
 
 	// TODO: Is there any way to avoid doing this wait? Lease TTL granularity is in seconds.
 	select {