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. - 

if ((*curr_jt)->info.partition_id != (*prev_jt)->info.partition_id)
            {
                ++prev_jt;
                ++curr_jt;
                continue;
            }
            if ((*curr_jt)->contains(**prev_jt))
            {
                deactivate_part(prev_jt);
                prev_jt = curr_jt;
                ++curr_jt;
            }
            else if ((*prev_jt)->contains(**curr_jt))
            {
                auto next = std::next(curr_jt);
                deactivate_part(curr_jt);
                curr_jt = next;
            }
            else
            {
                ++prev_jt;
                ++curr_jt;
            }
        }
    }
    calculateColumnSizesImpl();
    LOG_DEBUG(log, "Loaded data parts ({} items)", data_parts_indexes.size());
}
static bool isOldPartDirectory(const DiskPtr & disk, const String & directory_path, time_t threshold)