channeldb+migration27: patch balance fields for historical chan

This commit adds a new migration to patch the two balance fields,
`InitialLocalBalance` and `InitialRemoteBalance` for the historical
channels. Because they are not saved previously, for historical channels
prior to the revocation log PR, these fields will be empty.

channeldb/db.go

@@ -21,6 +21,7 @@ import (
 	"github.com/lightningnetwork/lnd/channeldb/migration24"
 	"github.com/lightningnetwork/lnd/channeldb/migration25"
 	"github.com/lightningnetwork/lnd/channeldb/migration26"
+	"github.com/lightningnetwork/lnd/channeldb/migration27"
 	"github.com/lightningnetwork/lnd/channeldb/migration_01_to_11"
 	"github.com/lightningnetwork/lnd/clock"
 	"github.com/lightningnetwork/lnd/kvdb"
@@ -219,6 +220,12 @@ var (
 			number:    26,
 			migration: migration26.MigrateBalancesToTlvRecords,
 		},
+		{
+			// Patch the initial local/remote balance fields with
+			// empty values for historical channels.
+			number:    27,
+			migration: migration27.MigrateHistoricalBalances,
+		},
 	}
 
 	// Big endian is the preferred byte order, due to cursor scans over