ZGC: Don't track size in ZPhysicalMemoryBacking

To make it easier to allow for more concurrent commit/uncommit we should not track the backing "size" in ZPhysicalMemoryBacking. Instead we can add the full physical memory range (from 0 to max_capacity) to the list of uncommitted memory at startup. This removes the need to track and update the backing size (which otherwise needs to be properly synchronized).