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. - 

}
        }
    }
    checkProperties(new_metadata, old_metadata);
    checkTTLExpressions(new_metadata, old_metadata);
    if (!columns_to_check_conversion.empty())
    {
        auto old_header = old_metadata.getSampleBlock();
        performRequiredConversions(old_header, columns_to_check_conversion, getContext());
    }
    if (old_metadata.hasSettingsChanges())
    {
        const auto current_changes = old_metadata.getSettingsChanges()->as<const ASTSetQuery &>().changes;
        const auto & new_changes = new_metadata.settings_changes->as<const ASTSetQuery &>().changes;
        for (const auto & changed_setting : new_changes)
        {
            const auto & setting_name = changed_setting.name;
            const auto & new_value = changed_setting.value;
            MergeTreeSettings::checkCanSet(setting_name, new_value);
            const Field * current_value = current_changes.tryGet(setting_name);
            if ((!current_value || *current_value != new_value)
                && MergeTreeSettings::isReadonlySetting(setting_name))
            {
                throw Exception{"Setting '" + setting_name + "' is readonly for storage '" + getName() + "'",
                                 ErrorCodes::READONLY_SETTING};
            }
            if (!current_value && MergeTreeSettings::isPartFormatSetting(setting_name))
            {
                MergeTreeSettings copy = *getSettings();
                copy.applyChange(changed_setting);