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_entries.push_back(std::move(merge_entry));
                    break;
                }
                if (try_no == max_retries)
                    return handle_noop("Can't create merge queue node in ZooKeeper, because log was updated in every of "
                        + toString(max_retries) + " tries");
            }
        }
        else
        {
            size_t try_no = 0;
            for (; try_no < max_retries; ++try_no)
            {
                std::lock_guard merge_selecting_lock(merge_selecting_mutex);
                ReplicatedMergeTreeMergePredicate can_merge = queue.getMergePredicate(zookeeper);
                FutureMergedMutatedPart future_merged_part;
                if (storage_settings.get()->assign_part_uuids)
                    future_merged_part.uuid = UUIDHelpers::generateV4();
                String disable_reason;
                SelectPartsDecision select_decision = SelectPartsDecision::CANNOT_SELECT;
                if (!partition)
                {
                    select_decision = merger_mutator.selectPartsToMerge(
                        future_merged_part, true, storage_settings_ptr->max_bytes_to_merge_at_max_space_in_pool, can_merge, false, &disable_reason);
                }
                else
                {
                    UInt64 disk_space = getStoragePolicy()->getMaxUnreservedFreeSpace();
                    String partition_id = getPartitionIDFromQuery(partition, query_context);
                    select_decision = merger_mutator.selectAllPartsToMergeWithinPartition(