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 context,
        TableLockHolder & table_lock_holder);
    
    PartitionCommandsResultInfo freezeAll(
        const String & with_name,
        const StorageMetadataPtr & metadata_snapshot,
        ContextPtr context,
        TableLockHolder & table_lock_holder);
    
    PartitionCommandsResultInfo unfreezePartition(
        const ASTPtr & partition,
        const String & backup_name,
        ContextPtr context,
        TableLockHolder & table_lock_holder);
    
    PartitionCommandsResultInfo unfreezeAll(
        const String & backup_name,
        ContextPtr context,
        TableLockHolder & table_lock_holder);
    
    void movePartitionToDisk(const ASTPtr & partition, const String & name, bool moving_part, ContextPtr context);
    
    void movePartitionToVolume(const ASTPtr & partition, const String & name, bool moving_part, ContextPtr context);
    void checkPartitionCanBeDropped(const ASTPtr & partition) override;
    void checkPartCanBeDropped(const ASTPtr & part);
    Pipe alterPartition(
        const StorageMetadataPtr & metadata_snapshot,
        const PartitionCommands & commands,
        ContextPtr query_context) override;
    size_t getColumnCompressedSize(const std::string & name) const