**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.
if (!next_block_in_current_unit.has_value())
    {
        
        
        std::unique_lock<std::mutex> lock(mutex);
        reader_condvar.wait(lock, [&](){ return unit.status == READY_TO_READ || finished; });
        if (finished)
        {
            /**
              * Check for background exception and rethrow it before we return.
              */
            if (background_exception)
            {
                lock.unlock();
                cancel(false);
                std::rethrow_exception(background_exception);
            }
            return Block{};
        }
        assert(unit.status == READY_TO_READ);
        next_block_in_current_unit = 0;
    }
    if (unit.block_ext.block.empty())
    {
        /*
         * Can we get zero blocks for an entire segment, when the format parser
         * skips it entire content and does not create any blocks? Probably not,
         * but if we ever do, we should add a loop around the above if, to skip
         * these. Also see a matching assert in the parser thread.
         */