chainio: implement Blockbeat

In this commit, a minimal implementation of `Blockbeat` is added to
synchronize block heights, which will be used in `ChainArb`, `Sweeper`,
and `TxPublisher` so blocks are processed sequentially among them.

chainio/blockbeat_test.go

@@ -0,0 +1,28 @@
+package chainio
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/lightningnetwork/lnd/chainntnfs"
+	"github.com/stretchr/testify/require"
+)
+
+var errDummy = errors.New("dummy error")
+
+// TestNewBeat tests the NewBeat and Height functions.
+func TestNewBeat(t *testing.T) {
+	t.Parallel()
+
+	// Create a testing epoch.
+	epoch := chainntnfs.BlockEpoch{
+		Height: 1,
+	}
+
+	// Create the beat and check the internal state.
+	beat := NewBeat(epoch)
+	require.Equal(t, epoch, beat.epoch)
+
+	// Check the height function.
+	require.Equal(t, epoch.Height, beat.Height())
+}