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 AlterCommands & commands,
    ContextPtr local_context,
    TableLockHolder & table_lock_holder)
{
    auto table_id = getStorageID();
    auto old_storage_settings = getSettings();
    StorageInMemoryMetadata new_metadata = getInMemoryMetadata();
    StorageInMemoryMetadata old_metadata = getInMemoryMetadata();
    auto maybe_mutation_commands = commands.getMutationCommands(new_metadata, local_context->getSettingsRef().materialize_ttl_after_modify, local_context);
    String mutation_file_name;
    Int64 mutation_version = -1;
    commands.apply(new_metadata, local_context);
    
    if (commands.isSettingsAlter())
    {
        changeSettings(new_metadata.settings_changes, table_lock_holder);
        DatabaseCatalog::instance().getDatabase(table_id.database_name)->alterTable(local_context, table_id, new_metadata);
    }