Add a builder to TransactionHelper for creating the needed transactional context

We often have the case we want to commit the current transaction and start a new one (generally a longer one), but we're missing convenient method in TransactionHelper to do that.

We will benefit from this common mechanism by adding the TransactionHelper#runInSeparateTransaction methods which commit the current transaction (if exist), start a new one, do the work, commit the later, and eventually start a new transaction.