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)