Work in progree: Changed layout of GenericObject::asyncLockfreeFetch so it...

Work in progree: Changed layout of GenericObject::asyncLockfreeFetch so it matches the model of previous sync operations

like ArchiveRequest::asyncUpdateJobOwner().
Also made the no lock management match the model of previously developped nolock operations.
Removed Backend::AsyncLockfreeFetcher::wait() as get() will pass the exceptions anyway.