Further partial implementation of Retrieve request queuing

objectstore/ArchiveQueue.cpp

objectstore/ArchiveRequest.cpp

objectstore/ArchiveRequest.hpp

objectstore/GarbageCollectorTest.cpp

objectstore/ProtocolBuffersAlgorithms.hpp

objectstore/RetrieveQueue.cpp

objectstore/RetrieveQueue.hpp

objectstore/RetrieveQueueTest.cpp

objectstore/RetrieveRequest.cpp

objectstore/RetrieveRequest.hpp

objectstore/RootEntry.cpp

objectstore/cta.proto

scheduler/OStoreDB/OStoreDB.cpp

scheduler/OStoreDB/OStoreDB.hpp

scheduler/OStoreDB/OStoreDBFactory.hpp

scheduler/Scheduler.cpp

scheduler/SchedulerDatabase.hpp

scheduler/SchedulerTest.cpp