Introduce reasonable transaction boundaries for multi query operations

[meistermeier]

Be it save with relationships or loading of cyclic graph data, both operations will issue multiple queries.
If the user does not specify transactions via @Transactional or TransactionTemplate/TransactionalOperator, each query will happen in its own transaction. To avoid this surprising scenario for the users we should create transactions around those batches of operations if the user does not specify a broader transactional scope.