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. - 

merge_selecting_task->schedule();
    }
}
void StorageReplicatedMergeTree::mutationsFinalizingTask()
{
    bool needs_reschedule = false;
    try
    {
        needs_reschedule = queue.tryFinalizeMutations(getZooKeeper());
    }
    catch (...)
    {
        tryLogCurrentException(log, __PRETTY_FUNCTION__);
        needs_reschedule = true;
    }
    if (needs_reschedule)
    {
        mutations_finalizing_task->scheduleAfter(MUTATIONS_FINALIZING_SLEEP_MS);
    }
    else
    {
        
        
        
        
        
        mutations_finalizing_task->scheduleAfter(MUTATIONS_FINALIZING_IDLE_SLEEP_MS);
    }
}
StorageReplicatedMergeTree::CreateMergeEntryResult StorageReplicatedMergeTree::createLogEntryToMergeParts(