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. - 

deduplicate_by_columns,
                    &disable_reason,
                    local_context->getSettingsRef().optimize_skip_merged_partitions))
            {
                constexpr const char * message = "Cannot OPTIMIZE table: {}";
                if (disable_reason.empty())
                    disable_reason = "unknown reason";
                LOG_INFO(log, message, disable_reason);
                if (local_context->getSettingsRef().optimize_throw_if_noop)
                    throw Exception(ErrorCodes::CANNOT_ASSIGN_OPTIMIZE, message, disable_reason);
                return false;
            }
        }
    }
    else
    {
        String partition_id;
        if (partition)
            partition_id = getPartitionIDFromQuery(partition, local_context);
        if (!merge(
                true,
                partition_id,
                final,
                deduplicate,
                deduplicate_by_columns,
                &disable_reason,
                local_context->getSettingsRef().optimize_skip_merged_partitions))
        {
            constexpr const char * message = "Cannot OPTIMIZE table: {}";
            if (disable_reason.empty())