[hooks] Do not register "logstats" if repository has no scheduler

[server] Introduce an `has_scheduler` method on Repository This is to be used by client application to determine if looping tasks may be registered in the current process. By checking this, one will avoid the warning in looping_task method when the repository has no scheduler.

[server] Warn instead of failing when a looping task is registered and repo has no scheduler We should provide a way for client code to detect if they should register the looping task or not. See next patch for that.