Change the memory space so that it only includes a single memory resource

[felipeblazing]

After discussions we concluded that a memory_space need not have more than one memory_resource available. Previously we had been thinking that a Memory Space will map to just a Tier and device_id, where device_id can be something like a numa node, a gpu device id, or an id that maps to a file path.

Now we think that want to distinguish between pinned and pageable as different memory spaces on host then it really only makes sense to have a single memory resource per space.