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. - 

ContextPtr query_context,
    QueryProcessingStage::Enum to_stage,
    const StorageMetadataPtr & metadata_snapshot,
    SelectQueryInfo & query_info) const
{
    if (to_stage >= QueryProcessingStage::Enum::WithMergeableState)
    {
        if (getQueryProcessingStageWithAggregateProjection(query_context, metadata_snapshot, query_info))
        {
            if (query_info.projection->desc->type == ProjectionDescription::Type::Aggregate)
                return QueryProcessingStage::Enum::WithMergeableState;
        }
    }
    return QueryProcessingStage::Enum::FetchColumns;
}
MergeTreeData & MergeTreeData::checkStructureAndGetMergeTreeData(IStorage & source_table, const StorageMetadataPtr & src_snapshot, const StorageMetadataPtr & my_snapshot) const