1) move AbortOption from NdbTransaction to NdbOperation
2) let each operation have a "default" abort option dependant on
operation type
- read - AO_IgnoreError
- dml - AbortOnError
- scan take over - AbortOnError
3) Changed default value to execute() from AbortOnError to DefaultAbortOption, which does not change the operations abort-option.
Another value to execute(AO) is equivalent to setting AO on each operation before calling execute
4) execute() does _only_ return -1 if transaction has been aborted
otherwise, you need to check each operation for error code
