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. - 

void checkAlterIsPossible(const AlterCommands & commands, ContextPtr context) const override;
    
    
    void checkMutationIsPossible(const MutationCommands & commands, const Settings & settings) const override;
    
    void checkAlterPartitionIsPossible(const PartitionCommands & commands, const StorageMetadataPtr & metadata_snapshot, const Settings & settings) const override;
    
    void changeSettings(
        const ASTPtr & new_settings,
        TableLockHolder & table_lock_holder);
    
    void reportBrokenPart(const String & name) const
    {
        broken_part_callback(name);
    }
    
    
    static ASTPtr extractKeyExpressionList(const ASTPtr & node)
    {
        return DB::extractKeyExpressionList(node);
    }
    /** Create local backup (snapshot) for parts with specified prefix.
      * Backup is created in directory clickhouse_dir/shadow/i/, where i - incremental number,
      *  or if 'with_name' is specified - backup is created in directory with specified name.
      */
    PartitionCommandsResultInfo freezePartition(
        const ASTPtr & partition,
        const StorageMetadataPtr & metadata_snapshot,
        const String & with_name,