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 settings = getSettings();
    if (settings->non_replicated_deduplication_window != 0 && format_version < MERGE_TREE_DATA_MIN_FORMAT_VERSION_WITH_CUSTOM_PARTITIONING)
        throw Exception("Deduplication for non-replicated MergeTree in old syntax is not supported", ErrorCodes::BAD_ARGUMENTS);
    std::string path = getDataPaths()[0] + "/deduplication_logs";
    deduplication_log = std::make_unique<MergeTreeDeduplicationLog>(path, settings->non_replicated_deduplication_window, format_version);
    deduplication_log->load();
}
void StorageMergeTree::loadMutations()