DATAJPA-601 - Improve performance with Propagation.SUPPORTS for readOnly operation.

Changed default propagation for read-only operations from REQUIRED to SUPPORTS.
This avoids creating unnecessary transaction synchronisations.

This change uses a different code path in than before. We now hit the case:
"// Create "empty" transaction" in which avoids the creation of a new tx.
org.springframework.transaction.support.AbstractPlatformTransactionManager.getTransaction(TransactionDefinition).

I verified this with an external example app.