transaction: Use Atomic to store/load the status

Transactions save the status of each operation in a status field, however
such field could be written concurrently by various operations, so we
need to be sure that:
 - We always return the status for the current operation
 - We store the status in a atomic way so that other actions won't
   create write races

In general, in a multi-thread operation one should not rely on
Transaction.Error() to get info about the last operation.

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