Add device defrag functionality to ring

A defrag capability would allow device 'holes' to be reused without affecting device ordering. This allows the oldest device in ring to be first one in list and newest device to be last non-empty one. Ordering of the devices will remain unchanged. However, the devices get packed contiguously and all empty device slots are moved to end of the list. Having devices remain in chronological order can make it easier to track age within ring.