**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.
{
private:
    using ReadCallback = std::function<void()>;
    using InputProcessorCreator = std::function<InputFormatPtr(
            ReadBuffer & buf,
            const Block & header,
            const RowInputFormatParams & params,
            const FormatSettings & settings)>;
public:
    struct InputCreatorParams
    {
        const Block & sample;
        const RowInputFormatParams & row_input_format_params;
        const FormatSettings &settings;
    };
    struct Params
    {
        ReadBuffer & read_buffer;
        const InputProcessorCreator & input_processor_creator;
        const InputCreatorParams & input_creator_params;
        FormatFactory::FileSegmentationEngine file_segmentation_engine;
        int max_threads;
        size_t min_chunk_bytes;
    };
    explicit ParallelParsingBlockInputStream(const Params & params)