channeldb/migration13: migrate to mpp structure

This commit migrates the payments in the database to a new structure
that allows for multiple htlcs per payments. The migration introduces a
new sub-bucket that contains a list of htlcs and moves the old single
htlc into that.

channeldb/db.go

@@ -14,6 +14,7 @@ import (
 	"github.com/coreos/bbolt"
 	"github.com/go-errors/errors"
 	"github.com/lightningnetwork/lnd/channeldb/migration12"
+	"github.com/lightningnetwork/lnd/channeldb/migration13"
 	"github.com/lightningnetwork/lnd/channeldb/migration_01_to_11"
 	"github.com/lightningnetwork/lnd/clock"
 	"github.com/lightningnetwork/lnd/lnwire"
@@ -124,6 +125,11 @@ var (
 			number:    12,
 			migration: migration12.MigrateInvoiceTLV,
 		},
+		{
+			// Migrate to multi-path payments.
+			number:    13,
+			migration: migration13.MigrateMPP,
+		},
 	}
 
 	// Big endian is the preferred byte order, due to cursor scans over