kernel: Protect scheduler_set_thread_priority() with lock
kernel/util: Fix MinMaxHeap::_GrowHeap()
scheduler_affine: Disable logic not needed on current topology
scheduler: Remove support for running different schedulers

Simple scheduler behaves exactly the same as affine scheduler with a
single core. Obviously, affine scheduler is more complicated thus
introduces greater overhead but quite a lot of multicore logic has been
disabled on single core systems in the previous commit.