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. - 

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;
        }
    }
    return true;
}
ActionLock StorageMergeTree::stopMergesAndWait()
{
    
    
    auto merge_blocker = merger_mutator.merges_blocker.cancel();
    {
        std::unique_lock lock(currently_processing_in_background_mutex);
        while (!currently_merging_mutating_parts.empty())
        {
            LOG_DEBUG(log, "Waiting for currently running merges ({} parts are merging right now)",
                currently_merging_mutating_parts.size());
            if (std::cv_status::timeout == currently_processing_in_background_condition.wait_for(
                lock, std::chrono::seconds(DBMS_DEFAULT_LOCK_ACQUIRE_TIMEOUT_SEC)))
            {
                throw Exception("Timeout while waiting for already running merges", ErrorCodes::TIMEOUT_EXCEEDED);
            }
        }
    }
    return merge_blocker;
}
MergeTreeDataPartPtr StorageMergeTree::outdatePart(const String & part_name, bool force)