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. - 

StorageMergeTree::write(const ASTPtr & , const StorageMetadataPtr & metadata_snapshot, ContextPtr local_context)
{
    const auto & settings = local_context->getSettingsRef();
    return std::make_shared<MergeTreeBlockOutputStream>(
        *this, metadata_snapshot, settings.max_partitions_per_insert_block, local_context);
}
void StorageMergeTree::checkTableCanBeDropped() const
{
    auto table_id = getStorageID();
    getContext()->checkTableCanBeDropped(table_id.database_name, table_id.table_name, getTotalActiveSizeInBytes());
}
void StorageMergeTree::drop()
{
    shutdown();
    dropAllData();
}
void StorageMergeTree::truncate(const ASTPtr &, const StorageMetadataPtr &, ContextPtr, TableExclusiveLockHolder &)
{
    {
        
        
        auto merge_blocker = stopMergesAndWait();
        auto parts_to_remove = getDataPartsVector();
        removePartsFromWorkingSet(parts_to_remove, true);
        LOG_INFO(log, "Removed {} parts.", parts_to_remove.size());
    }
    clearOldMutations(true);
    clearOldPartsFromFilesystem();
}
void StorageMergeTree::alter(