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. - 

#include <Core/Defines.h>
#include <Common/Macros.h>
#include <Common/StringUtils/StringUtils.h>
#include <Common/ThreadPool.h>
#include <Common/ZooKeeper/KeeperException.h>
#include <Common/ZooKeeper/Types.h>
#include <Common/escapeForFileName.h>
#include <Common/formatReadable.h>
#include <Common/thread_local_rng.h>
#include <Common/typeid_cast.h>
#include <Storages/AlterCommands.h>
#include <Storages/PartitionCommands.h>
#include <Storages/ColumnsDescription.h>
#include <Storages/StorageReplicatedMergeTree.h>
#include <Storages/MergeTree/IMergeTreeDataPart.h>