Make FetchLeavesFromFile use IDataManagerSvc::traverseSubTree
Some implementations of an event transient store may not have implemented all methods of IDataManagerSvc and in LHCb we have a case where we have implemented IDataManagerSvc::traverseSubTree but not IDataManagerSvc::objectLeaves.
Since traverseSubTree is more used than objectLeaves it makes sense to convert FetchLeavesFromFile to use traverseSubTree instead of using a recursive method.