routing+channeldb: migrate MC store to use minimal Route encoding

Add a new mcRoute type that houses the data about a route that MC
actually uses. Then add a migration (channeldb/migration32) that
migrates the existing store from its current serialisation to the new,
more minimal serialisation.

channeldb/db.go

@@ -26,6 +26,7 @@ import (
 	"github.com/lightningnetwork/lnd/channeldb/migration29"
 	"github.com/lightningnetwork/lnd/channeldb/migration30"
 	"github.com/lightningnetwork/lnd/channeldb/migration31"
+	"github.com/lightningnetwork/lnd/channeldb/migration32"
 	"github.com/lightningnetwork/lnd/channeldb/migration_01_to_11"
 	"github.com/lightningnetwork/lnd/clock"
 	"github.com/lightningnetwork/lnd/invoices"
@@ -286,6 +287,10 @@ var (
 			number:    31,
 			migration: migration31.DeleteLastPublishedTxTLB,
 		},
+		{
+			number:    32,
+			migration: migration32.MigrateMCRouteSerialisation,
+		},
 	}
 
 	// optionalVersions stores all optional migrations that are applied