Added logging for agent ownership addition/removal.

common/log/LogContext.hpp

objectstore/AgentReference.cpp

objectstore/AgentReference.hpp

objectstore/BackendPopulator.cpp

objectstore/GarbageCollectorTest.cpp

objectstore/RetrieveQueueTest.cpp

objectstore/RootEntryTest.cpp

objectstore/cta-objectstore-collect-orphaned-object.cpp

objectstore/cta-objectstore-initialize.cpp

scheduler/OStoreDB/OStoreDBFactory.hpp