TxUtils.isActive does perform the expected check (except for UserTransactions).

Additionally, we need to distinguish the three different states.

• Active - the transaction is still running and work can be done (enlist/registerSynchronization is allowed)
• Uncommitted - the transaction is still running or it is marked rolledback, but 2PC has not yet started (delistResource should still be done)
• Completed - the transaction has been committed/rolledback (there is no transaction)