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