Fix a couple of minor issues with IOR output

[hjelmn]

This commit fixes two issues:

• The AIO backend prints the max outstanding transactions when starting. This is printed by every MPI rank and doesn't add much. Probably is left over from some debugging.

• Init MPI before parsing the command line. Without this every rank thinks it is rank 0 so messages that should only be printed by rank 0 are printed n times.

[hjelmn]

Been a long time since I touched IOR but these two issues have been bugging me. Julian, PTAL.

[hjelmn]

The only issue is the whole parse for -h before starting MPI thing. I can probably work around that if we want to maintain that. Honestly, starting MPI as a singleton is basically free so not a big deal.

[JulianKunkel]

The issue with -h is that there exist some MPI implementations that require a parallel job to be started before they can be initialized. Thus, it would not be possible to see the arguments of ior without running it in a parallel job.
This in turn might not be possible in an interactive way.
It is very annoying, therefore, I would not want to merge the "-h" behavior change.
An alternative might be that we have a "ior-help" tool or such without MPI, but it feels awkward. Any suggestions are welcome though.