Make LRU manage aware of lazy device->host transfers

@robeverest I recently updated the PTX backend to only transfer results back to the host as they are demanded:

https://github.com/AccelerateHS/accelerate-llvm/blob/master/accelerate-llvm-ptx/Data/Array/Accelerate/LLVM/PTX/Array/Data.hs#L86

We should probably make the memory manager aware of this feature; what do you think is the best way to do this?