Add statistics according to #248

This add basic statistics for VRFs, pools and prefixes in the backend, exposes it via the API and enables support in pynipap so that the CLI and the web can display statistics.
I think it's simply the comment that is wrong. Example https://github.com/SpriteLink/NIPAP/wiki/Trigger-trickiness
The new prefix will have indent 1 and start to cover prefixes that previously had indent = 1. They WILL have NEW.indent+1 after the transaction is complete and all triggers have been run but since this is the before trigger, the other rows are still in their OLD state, thus their indent is same as NEW.indent.