multi: pass in a MigrationConfig to all optional migrations

Define a MigrationConfig interface that should be used to pass the
config of an optional migration function. An implementation of this
interface is added for migration30 (the only current optional
migration).

channeldb/migtest/migtest.go

@@ -89,7 +89,8 @@ func ApplyMigration(t *testing.T,
 // function. This function differs from ApplyMigration as it requires the
 // supplied migration functions to take a db instance and construct their own
 // database transactions.
-func ApplyMigrationWithDB(t testing.TB, beforeMigration, afterMigration,
+func ApplyMigrationWithDB(t testing.TB, beforeMigration,
+	afterMigration func(db kvdb.Backend) error,
 	migrationFunc func(db kvdb.Backend) error, shouldFail bool) {
 
 	t.Helper()