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 lock1 = lockForShare(local_context->getCurrentQueryId(), local_context->getSettingsRef().lock_acquire_timeout);
    auto lock2 = source_table->lockForShare(local_context->getCurrentQueryId(), local_context->getSettingsRef().lock_acquire_timeout);
    auto source_metadata_snapshot = source_table->getInMemoryMetadataPtr();
    auto my_metadata_snapshot = getInMemoryMetadataPtr();
    Stopwatch watch;
    MergeTreeData & src_data = checkStructureAndGetMergeTreeData(source_table, source_metadata_snapshot, my_metadata_snapshot);
    String partition_id = getPartitionIDFromQuery(partition, local_context);
    DataPartsVector src_parts = src_data.getDataPartsVectorInPartition(MergeTreeDataPartState::Committed, partition_id);
    MutableDataPartsVector dst_parts;
    static const String TMP_PREFIX = "tmp_replace_from_";
    for (const DataPartPtr & src_part : src_parts)
    {
        if (!canReplacePartition(src_part))
            throw Exception(
                "Cannot replace partition '" + partition_id + "' because part '" + src_part->name + "' has inconsistent granularity with table",
                ErrorCodes::BAD_ARGUMENTS);
        
        Int64 temp_index = insert_increment.get();
        MergeTreePartInfo dst_part_info(partition_id, temp_index, temp_index, src_part->info.level);
        dst_parts.emplace_back(cloneAndLoadDataPartOnSameDisk(src_part, TMP_PREFIX, dst_part_info, my_metadata_snapshot));
    }
    
    if (!replace && dst_parts.empty())
        return;
    MergeTreePartInfo drop_range;
    if (replace)
    {