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. - 

{
    bool prefixed = false;
    String id;
    if (format_version < MERGE_TREE_DATA_MIN_FORMAT_VERSION_WITH_CUSTOM_PARTITIONING)
    {
        /
        if (const auto * partition_lit = partition_ast->as<ASTPartition &>().value->as<ASTLiteral>())
        {
            id = partition_lit->value.getType() == Field::Types::UInt64
                 ? toString(partition_lit->value.get<UInt64>())
                 : partition_lit->value.safeGet<String>();
            prefixed = true;
        }
        else
            id = getPartitionIDFromQuery(partition_ast, local_context);
    }
    else
        id = getPartitionIDFromQuery(partition_ast, local_context);
    return [prefixed, id](const String & partition_id)
    {
        if (prefixed)
            return startsWith(partition_id, id);
        else
            return id == partition_id;
    };
}
PartitionCommandsResultInfo MergeTreeData::freezePartition(