[server] replace repository's tasks manager by a scheduler
Repository() does not accept anymore a 'tasks_manager' argument but rather a
'scheduler' argument which is expected to be an instance of sched.scheduler
class. The drop the _tasks_manager attribute of the repository and adjust all
internal usages of it. In particular, in the 'repo_stats' service we do not
export 'looping_tasks' statistics anymore as there's no way to retrieve this
anymore from a web instance.
Closes #17057223.