wrap all curl pointers in unique pointers for move correctness. the
destructor of TransferItem can go away completely since it does not
do anything special any more: memory bookkeeping is done using raii
wrappers, and its effects on the transfer state are not observable.
only TransferSource inspects the result, and even it does not check
for transfer results in its own destructor (only during transfers).
Change-Id: Ibb58484703854d5d5425e7c63cf2985d93920e97
eldritch horrors
4ba19f68fb