routing/shards: add ShardTracker interface

We'll use this to keep track of the outstanding shards and which
preimages we are using for each. For now this is a simple map from
attempt ID to hash, but later we'll hide the AMP child derivation behind
this interface.

routing/shards/shard_tracker_test.go

@@ -0,0 +1,47 @@
+package shards_test
+
+import (
+	"crypto/rand"
+	"testing"
+
+	"github.com/lightningnetwork/lnd/lntypes"
+	"github.com/lightningnetwork/lnd/routing/shards"
+	"github.com/stretchr/testify/require"
+)
+
+// TestSimpleShardTracker tests that the simple tracker that keeps a map from
+// attemptID-> payment hash works.
+func TestSimpleShardTracker(t *testing.T) {
+	var testHashes [2]lntypes.Hash
+	for i := range testHashes {
+		_, err := rand.Read(testHashes[i][:])
+		require.NoError(t, err)
+	}
+
+	startAttempts := map[uint64]lntypes.Hash{
+		1: testHashes[1],
+	}
+
+	tracker := shards.NewSimpleShardTracker(testHashes[0], startAttempts)
+
+	// Trying to retrieve a hash for id 0 should result in an error.
+	_, err := tracker.GetHash(0)
+	require.Error(t, err)
+
+	// Getting id 1 should workd.
+	hash1, err := tracker.GetHash(1)
+	require.NoError(t, err)
+
+	require.Equal(t, testHashes[1], hash1)
+
+	// Finally, create a new shard and immediately retrieve the hash.
+	shard, err := tracker.NewShard(2, false)
+	require.NoError(t, err)
+
+	// It's hash should be the tracker's overall payment hash.
+	hash2, err := tracker.GetHash(2)
+	require.NoError(t, err)
+
+	require.Equal(t, testHashes[0], shard.Hash())
+	require.Equal(t, testHashes[0], hash2)
+}