athenaHLT: Write histograms into their own directory when forking

- Add the missing call to `updateDFProps` after the fork to
  individualize the child properties (e.g. app name).
- Use the `IoComponentMgr` to mange the I/O for child processes as done in
  athenaMP. This creates a sub-directory for each child into which histogram
  files are being written.
- With the above we can run also in `athenaHLT.py` using the offline
  `THistSvc`.

TODO: It's not nice that we are creating empty directories in case the
online `THistSvc` is used. Can probably add a check on the number of
registered files and if zero, don't bother with the I/O component
manager.