sqldb+config: validate maximum batch size config value

Now that the SQL query config values are configurable, we add some
validation to make sure that the user doesnt set a max batch size that
is larger than the limits for sqlite/postgres that have been determined
by the TestSQLSliceQueries test.

lncfg/db.go

@@ -143,6 +143,9 @@ func (db *DB) Validate() error {
 		}
 
 	case SqliteBackend:
+		if err := db.Sqlite.Validate(); err != nil {
+			return err
+		}
 	case PostgresBackend:
 		if err := db.Postgres.Validate(); err != nil {
 			return err