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.
Hm, the comment states ```... the part that is now covered by NEW.prefix but wasn't covered by OLD.prefix will have NEW.indent+1.```, but the code reads ```indent = NEW.indent```.