Set transaction isolation level to "serializable"

Sda2/P/account/src/main/java/de/hdm_stuttgart/mi/sda2/account/sql/DbHandler.java

@@ -28,6 +28,7 @@ public class DbHandler {
 					Conf.get("DbHandler.user"),
 					Conf.get("DbHandler.password"));
 			conn.setAutoCommit(false);
+			conn.setTransactionIsolation(Connection.TRANSACTION_SERIALIZABLE);
 			updateAccount = conn.prepareStatement("UPDATE Account SET balance = balance + ? WHERE number = ?");
 			log.info("Connection '" + getConnectionName() + "' established" ); 
 			return true;