Add support for running multiple new SharedWriter instances in parallel - Part 2
This MR is a follow-up of !43079 (merged) and relates to ATEAM-739. In the original implementation initializing the AthenaRootSharedWriterSvc in the mother process had the side effect of corrupting metadata under some circumstances. Here we aim fixing that.
Let me cc @gemmeren for his valuable insight and keep this as Draft until details are ironed out.