Symptoms: all threads in BackgroundScheduledPool busy, lot of tasks in replication queue, none of the tasks is executing. 

![Screenshot from 2021-04-16 16-21-44_](https://user-images.githubusercontent.com/1549571/115193796-e767d600-a0ec-11eb-887c-5c9c2412e50c.png)

Related to #16895

Possible workarounds (poor options): increasing the background_schedule_pool_size to huge values, or restart server every time when that happens etc. - 

stopwatch.elapsed(),
            future_part.name,
            new_part,
            future_part.parts,
            merge_list_entry.get());
    };
    try
    {
        new_part = merger_mutator.mergePartsToTemporaryPart(
            future_part,
            metadata_snapshot,
            *(merge_list_entry),
            table_lock_holder,
            time(nullptr),
            getContext(),
            merge_mutate_entry.tagger->reserved_space,
            deduplicate,
            deduplicate_by_columns,
            merging_params);
        merger_mutator.renameMergedTemporaryPart(new_part, future_part.parts, nullptr);
        write_part_log({});
    }
    catch (...)
    {
        write_part_log(ExecutionStatus::fromCurrentException());
        throw;
    }
    return true;
}
bool StorageMergeTree::partIsAssignedToBackgroundOperation(const DataPartPtr & part) const