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. - 

#pragma once
#include <Common/SimpleIncrement.h>
#include <Common/MultiVersion.h>
#include <Storages/IStorage.h>
#include <Storages/MergeTree/MergeTreeIndices.h>
#include <Storages/MergeTree/MergeTreePartInfo.h>
#include <Storages/MergeTree/MergeTreeSettings.h>
#include <Storages/MergeTree/MergeTreeMutationStatus.h>
#include <Storages/MergeTree/MergeList.h>
#include <Storages/DataDestinationType.h>
#include <IO/ReadBufferFromString.h>
#include <IO/WriteBufferFromFile.h>
#include <IO/ReadBufferFromFile.h>
#include <DataTypes/DataTypeString.h>
#include <DataTypes/DataTypesNumber.h>
#include <Processors/Merges/Algorithms/Graphite.h>
#include <Storages/MergeTree/IMergeTreeDataPart.h>