message: lock around getAllocatedMiniheapCount
diff: @@ -76,6 +76,7 @@ public:
  void *allocFromArena(size_t sz);
  + // must be called with exclusive _mhRWLock held
  inline MiniHeap *allocMiniheap(int sizeClass, size_t pageCount, size_t objectCount, size_t objectSize) {
  const size_t spanSize = HL::CPUInfo::PageSize * pageCount;
  d_assert(0 < spanSize);
  @@ -373,6 +374,7 @@ public:
  }
  size_t getAllocatedMiniheapCount() const {
  + std::shared_lock<std::shared_timed_mutex> sharedLock(_mhRWLock);
  return Super::bitmap().inUseCount();
  }
  