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. - 

{
    CheckResults results;
    DataPartsVector data_parts;
    if (const auto & check_query = query->as<ASTCheckQuery &>(); check_query.partition)
    {
        String partition_id = getPartitionIDFromQuery(check_query.partition, local_context);
        data_parts = getDataPartsVectorInPartition(MergeTreeDataPartState::Committed, partition_id);
    }
    else
        data_parts = getDataPartsVector();
    for (auto & part : data_parts)
    {
        try
        {
            results.push_back(part_check_thread.checkPart(part->name));
        }
        catch (const Exception & ex)
        {
            results.emplace_back(part->name, false, "Check of part finished with error: '" + ex.message() + "'");
        }
    }
    return results;
}
bool StorageReplicatedMergeTree::canUseAdaptiveGranularity() const