Import19 optimizations

[UlrichB22]

This issue is intended to document some optimizations of the import19 migration program.

1. the import19 command should have 'procs' and 'limitmb'-options to be used for the index-build step to improve performance.
2. tracebacks in the 'drop and rebuild index' step should be handled
3. when parent items for sub-items are missing ('These items have orphan sub-items'-message in the +index view), they should be created automatically.
4. ...