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. - 

const StorageMetadataPtr & metadata_snapshot,
    const PartitionCommands & commands,
    ContextPtr query_context)
{
    PartitionCommandsResultInfo result;
    for (const PartitionCommand & command : commands)
    {
        PartitionCommandsResultInfo current_command_results;
        switch (command.type)
        {
            case PartitionCommand::DROP_PARTITION:
                if (command.part)
                    checkPartCanBeDropped(command.partition);
                else
                    checkPartitionCanBeDropped(command.partition);
                dropPartition(command.partition, command.detach, command.part, query_context);
                break;
            case PartitionCommand::DROP_DETACHED_PARTITION:
                dropDetached(command.partition, command.part, query_context);
                break;
            case PartitionCommand::ATTACH_PARTITION:
                current_command_results = attachPartition(command.partition, metadata_snapshot, command.part, query_context);
                break;
            case PartitionCommand::MOVE_PARTITION:
            {
                switch (*command.move_destination_type)
                {
                    case PartitionCommand::MoveDestinationType::DISK:
                        movePartitionToDisk(command.partition, command.move_destination_name, command.part, query_context);