[IMT] Add TThreadedObject::GetAtSlotRaw
[IMT] Add TThreadedObject::GetAtSlotRaw

Compared to `GetAtSlotUnchecked`, `GetAtSlotRaw` trades some more
safety for some more performance: not only it avoids checking for
slot initialization (as `GetAtSlotUnchecked` does) but it also avoids
construction of a `shared_ptr`.

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