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 <ext/shared_ptr_helper.h>
#include <atomic>
#include <pcg_random.hpp>
#include <Storages/IStorage.h>
#include <Storages/MergeTree/MergeTreeData.h>
#include <Storages/MergeTree/MergeTreeDataMergerMutator.h>
#include <Storages/MergeTree/MergeTreePartsMover.h>
#include <Storages/MergeTree/MergeTreeDataWriter.h>
#include <Storages/MergeTree/MergeTreeDataSelectExecutor.h>
#include <Storages/MergeTree/ReplicatedMergeTreeLogEntry.h>
#include <Storages/MergeTree/ReplicatedMergeTreeQueue.h>
#include <Storages/MergeTree/ReplicatedMergeTreeCleanupThread.h>
#include <Storages/MergeTree/ReplicatedMergeTreeRestartingThread.h>
#include <Storages/MergeTree/ReplicatedMergeTreeMergeStrategyPicker.h>
#include <Storages/MergeTree/ReplicatedMergeTreePartCheckThread.h>
#include <Storages/MergeTree/ReplicatedMergeTreeTableMetadata.h>
#include <Storages/MergeTree/EphemeralLockInZooKeeper.h>
#include <Storages/MergeTree/DataPartsExchange.h>
#include <Storages/MergeTree/ReplicatedMergeTreeAddress.h>
#include <Storages/MergeTree/LeaderElection.h>
#include <Storages/MergeTree/PartMovesBetweenShardsOrchestrator.h>
#include <DataTypes/DataTypesNumber.h>