Align statistics output in CLI

Output for viewing a VRF was different that pool and prefix so it has
now be modified to align all output of statistics data and make it more
compact. Further the actual "columns" are now aligned on the terminal
screen.

Part of #248.
Use exponent format for IPv6 statistics

Makes IPv6 statistics output a bit more compact, showing just how many
addresses are available becomes a next to incomprehensible long number.

Part of #248.
Rename child_prefixes_vX to used_addresses_vX

To conform with the naming of other columns / attributes for prefixes
and VRF.

Part of #248.