Improve pool statistics

Implement the improved pool statistics as per #528.

The idea behind this is to list the number of total / used / free prefixes instead of just listing total / used / free addresses. Since the default prefix-length of a pool is typically used when assigning new prefixes from that pool, it becomes more interesting with the amount of free prefixes than free addresses.
    
The number of used prefixes is defined as the number of direct childs to a member prefix of the pool. Free prefixes should be the number of free prefixes with the pool's default assignment size in all the member pool while the total prefixes is used + free combined.
    
This leads to the slightly awkward side-effect that the total number of prefixes in a pool can vary if non-default prefix-lengths are used.