Read IOV Condition objects using dedicated PersSvc instance

IOVSvc is requesting reading of its IOV Conditions objects into a dedicated PersistencySvc instance (context), so all conditions files can be closed by simply deleting this instance. This was up to now ignored by PoolSvc/ConversionSvc (by mistake). Here I am fixing this mistake and adding context-awarness to 2 converters that read objects containing POOL Refs to other object (DataHeaderCnv and CoolMultChanCnv). I also add EndProcessing incident trigger to IOVDbSvc because in AthenaMT the StoreCleared incident is not fired (this is to trigger Condition files closing after the first Event)