| Get a transaction. Following modes are supported:
- TX_REQUIRED: A new transaction is allocated if no current
transaction exists, otherwise the current transaction is returned.
- TX_NEW: A new transaction is allocated either way.
- TX_OPTIONAL: If there is a current transaction, that is returned,
null otherwise.
Note, that each transaction can support multiple levels of begin-commit
blocks. Each transaction only commits/rollsback is the most outer
block is commited/rolled back.
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