core/sched fixup and optimize

* fix some broken misuse of variables (mixing `tcb_t` and `pid_t`)
* fix condition logic (`X && Y != Y` -> `X && (Y != Y)`)
* fix setting and use of `thread_active_pid`, includes optimizations
* refactor `SCHEDSTATISTICS` to make it faster
* remove stale TODO and double spaces
What issue?

MODULE_HWTIMER is always built (i.e. not a module anymore as the commit message says) - so there's no need to model the dependency anymore.