Do not create SlotSets on the stack

[chakaz]

Function GetRemovedSlots() creates and returns (on the stack) a SlotSet of size 16kb. This can cause stack overflows.

[chakaz]

We should also probably move these to be private, if possible:

  SlotSet(const TBitSet& s) {
    *slots_ = s;
  }

  SlotSet(const SlotSet& s) {
    *slots_ = *s.slots_;
  }