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 query_context, TableLockHolder & table_lock_holder)
{
    assertNotReadonly();
    auto table_id = getStorageID();
    if (commands.isSettingsAlter())
    {
        
        
        StorageInMemoryMetadata future_metadata = getInMemoryMetadata();
        commands.apply(future_metadata, query_context);
        merge_strategy_picker.refreshState();
        changeSettings(future_metadata.settings_changes, table_lock_holder);
        DatabaseCatalog::instance().getDatabase(table_id.database_name)->alterTable(query_context, table_id, future_metadata);
        return;
    }
    auto ast_to_str = [](ASTPtr query) -> String
    {
        if (!query)
            return "";
        return queryToString(query);
    };
    const auto zookeeper = getZooKeeper();
    std::optional<ReplicatedMergeTreeLogEntryData> alter_entry;
    std::optional<String> mutation_znode;
    while (true)
    {
        
        alter_entry.emplace();
        mutation_znode.reset();
        auto current_metadata = getInMemoryMetadataPtr();