Support EvtStoreSvc in PackParticlesAndVertices
PackParticlesAndVertices used to use IDataManagerSvc::objectLeaves to traverse the TES and build its container list. This method is not
supported by all implementations of IDataManagerSvc (in particular
EvtStoreSvc). Instead, use IDataManagerSvc::traverseSubTree, which is
available in all implementations.
The newly-added ensure_prefix method is also to support EvtStoreSvc,
which does not guarantee that the IRegistry::identifier method of held
data objects returns a location prefixed with the event store root
location (typically /Event), see gaudi/Gaudi#162 (comment 4029205).
/cc @axu this patch should get the PersistReco tests a little further along.