watchtower: start using the new channel max heights

This commit also adds tests for the DB changes made in the previous
commit since we can now read the new field with the FetchChanInfos
method.

The commit following this one does the backfill migration.

watchtower/wtdb/queue_test.go

@@ -4,6 +4,7 @@ import (
 	"testing"
 
 	"github.com/lightningnetwork/lnd/kvdb"
+	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/watchtower/wtdb"
 	"github.com/stretchr/testify/require"
 )
@@ -31,6 +32,14 @@ func TestDiskQueue(t *testing.T) {
 		require.NoError(t, err)
 	})
 
+	// In order to test that the queue's `onItemWrite` call back (which in
+	// this case will be set to maybeUpdateMaxCommitHeight) is executed as
+	// expected, we need to register a channel so that we can later assert
+	// that it's max height field was updated properly.
+	var chanID lnwire.ChannelID
+	err = db.RegisterChannel(chanID, []byte{})
+	require.NoError(t, err)
+
 	namespace := []byte("test-namespace")
 	queue := db.GetDBQueue(namespace)
 
@@ -110,4 +119,19 @@ func TestDiskQueue(t *testing.T) {
 	// This should not have changed the order of the tasks, they should
 	// still appear in the correct order.
 	popAndAssert(task1, task2, task3, task4, task5, task6)
+
+	// Finally, we check that the `onItemWrite` call back was executed by
+	// the queue. We do this by checking that the channel's recorded max
+	// commitment height was set correctly. It should be equal to the height
+	// recorded in task6.
+	infos, err := db.FetchChanInfos()
+	require.NoError(t, err)
+	require.Len(t, infos, 1)
+
+	info, ok := infos[chanID]
+	require.True(t, ok)
+	require.True(t, info.MaxHeight.IsSome())
+	info.MaxHeight.WhenSome(func(height uint64) {
+		require.EqualValues(t, task6.CommitHeight, height)
+	})
 }