**Summary**
After upgrading a host that is just a collection `Distributed` tables to 20.4.X and 20.5.2.7, we observe the new `MemoryTracking` setting increasing substantially during writes, leading to individual writes and reads yielding exceptions: ```Memory limit (total) exceeded: would use 28.28 GiB (attempt to allocate chunk of 4197840 bytes), maximum: 28.28```

System metrics show that no actual memory is being allocated to the same size.
<img width="993" alt="Screen Shot 2020-07-19 at 12 23 59 PM" src="https://user-images.githubusercontent.com/1189446/87883721-229e7280-c9be-11ea-851b-0e1511cc10bb.png">

Please let me know if there is any additional context, diagrams, or statements I should provide. I'm interested in knowing how to correlate the total `MemoryTracking` metric to anything else since system memory does not reflect the metric at all.

**Context**
This host has four different `Distributed` tables that are written to via the HTTP interface with `insert_distributed_sync=1` set.
The host itself has no local tables.

The hosts with the downstream local tables are all 20.5.2.7.

We have downgraded the `Distributed` host in question back to 20.3.9.70 in order to continue writing to the clusters.
assert(unit.is_last);
        finished = true;
        return Block{};
    }
    assert(next_block_in_current_unit.value() < unit.block_ext.block.size());
    Block res = std::move(unit.block_ext.block.at(*next_block_in_current_unit));
    last_block_missing_values = std::move(unit.block_ext.block_missing_values[*next_block_in_current_unit]);
    next_block_in_current_unit.value() += 1;
    if (*next_block_in_current_unit == unit.block_ext.block.size())
    {
        
        next_block_in_current_unit.reset();
        ++reader_ticket_number;
        if (unit.is_last)
        {
            
            finished = true;
        }
        else
        {
            
            std::unique_lock<std::mutex> lock(mutex);
            unit.status = READY_TO_INSERT;
            segmentator_condvar.notify_all();
        }
    }
    return res;
}
}