AthenaPoolCnvSvc: Re-initialize AthenaPoolCnvSvc and extract the Pool Attributes for the output file
In AthenaMP, the POOL attributes are not propagated to the outputs of the workers. As a result, the ROOT settings for the files that come from the workers differ from the one that comes from the mother process (as well as serial Athena). Here, we make sure AthenaPoolCnvSvc gets re-initialized in the worker and we re-extract the output POOL attributes therein.
This change should help avoid the crash reported in ATEAM-679 because the branches will now have the correct/intended branch split-level of zero instead of 99.
I'm marking this as urgent as it would be nice to accept this before the Christmas break, in case @elmsheus wants to run another round of MC tests.