Method currentBytes

bolt/common/memory/MemoryPool.cpp:868–870

int64_t MemoryPoolImpl::releasableReservation() const {
  if (isLeaf()) {
    std::lock_guard<std::mutex> l(mutex_);
    return std::max<int64_t>(
        0, reservationBytes_ - quantizedSize(usedReservationBytes_));
  }
  if (reservedBytes() == 0) {
    return 0;