First refactoring of statistics classes (#6105)

First refactoring of statistics classes (#6105)

This PR does a first round of refactoring on the statistics, in particular the Stat class and derived classes.
It significantly shrinks the class hierarchy, modernizes some code (e.g. use std::chrono instead of clock_gettime), removes unused features (e.g. nesting of statistics) and does some general cleanup and consolidation.

Subsequent PRs are planned to change the ownership model (right now every module owns the Stat object) which makes the whole register / unregister mechanism obsolete.