Validated and fixed RetrieveRequest::garbageCollect with a unit test

Created a unit test for RetrieveRequest garbage collection.
Created a RetrieveQueue::addJobIfNecessary function.
Fixed various bugs in RetrieveRequest::garbageCollect.
Added basic functionnality to DummyCatalogue for GC unit tests.
Cleaned up interace of RetrieveRequest for functions that are never used.
Implemented missing function in RootEntry.
Homogenized capitalisation of object names.
Added support for RetrieveRequest in cta-objectstore-collect-orphaned-object.