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. - 

auto mutations_end_it = current_mutations_by_version.end();
    for (const auto & part : getDataPartsVector())
    {
        if (currently_merging_mutating_parts.count(part))
            continue;
        auto mutations_begin_it = current_mutations_by_version.upper_bound(part->info.getDataVersion());
        if (mutations_begin_it == mutations_end_it)
            continue;
        if (max_source_part_size < part->getBytesOnDisk())
        {
            LOG_DEBUG(
                log,
                "Current max source part size for mutation is {} but part size {}. Will not mutate part {} yet",
                max_source_part_size,
                part->getBytesOnDisk(),
                part->name);
            continue;
        }
        size_t current_ast_elements = 0;
        for (auto it = mutations_begin_it; it != mutations_end_it; ++it)
        {
            size_t commands_size = 0;
            MutationCommands commands_for_size_validation;
            for (const auto & command : it->second.commands)
            {
                if (command.type != MutationCommand::Type::DROP_COLUMN
                    && command.type != MutationCommand::Type::DROP_INDEX
                    && command.type != MutationCommand::Type::DROP_PROJECTION
                    && command.type != MutationCommand::Type::RENAME_COLUMN)
                {