FS: Extend SchedulingMode to intermediate queues

FS allows setting SchedulingMode for leaf queues. Extending this to non-leaf queues allows using different kinds of fairness: e.g., root can have three child queues - fair-mem, drf-cpu-mem, drf-cpu-disk-mem taking different number of resources into account. In turn, this allows users to decide on the scheduling latency vs sophistication of the scheduling mode.