Support promotion iterator in TinyLFU mmContainer #25

Currently we don't support promotion iterator (starting at the head of the the container) for the TinyLFU implementation. This iterator would start iterating over the items with the highest frequency first - then moving down the container to items with lower access frequency.

See MMTinyLFU.h/MMTinyLFU-inl.h for details:

cachelib/allocator/MMTinyLFU-inl.h line 235 at r4 (raw file):

void
MMTinyLFU::Container<T, HookPtr>::withPromotionIterator(F&& fun) {
  throw std::runtime_error("Not supported");