GMR1598: fix race condition in PersistencySvc::addCnvSvc
subset of gaudi/Gaudi!1598 (merged) (no updates to ref files or testing suite) to address race condition in PersistencySvc::addCnvSvc
subset of gaudi/Gaudi!1598 (merged) (no updates to ref files or testing suite) to address race condition in PersistencySvc::addCnvSvc