Different threads (e.g., RRC, DU tasks) might access the scheduler at
the same time as the thread that drivers the scheduler itself
(ru_thread). To avoid data races, introduce this mutex.

Most functions of the scheduler are only used internally and have been
marked static in the previous commit. The remaining ones, in this
commit, either lock the scheduler mutex sched_lock, or a comment has
been added of the assumption of how such function is to be locked (e.g.,
if it might be called from different places, or if it is called by a
function that already locks the scheduler).