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. - 

const StorageID & table_id_,
    const String & relative_data_path_,
    const StorageInMemoryMetadata & metadata_,
    ContextMutablePtr context_,
    const String & date_column_name,
    const MergingParams & merging_params_,
    std::unique_ptr<MergeTreeSettings> storage_settings_,
    bool require_part_metadata_,
    bool attach,
    BrokenPartCallback broken_part_callback_)
    : IStorage(table_id_)
    , WithMutableContext(context_->getGlobalContext())
    , merging_params(merging_params_)
    , require_part_metadata(require_part_metadata_)
    , relative_data_path(relative_data_path_)
    , broken_part_callback(broken_part_callback_)
    , log_name(table_id_.getNameForLogs())
    , log(&Poco::Logger::get(log_name))
    , storage_settings(std::move(storage_settings_))
    , pinned_part_uuids(std::make_shared<PinnedPartUUIDs>())
    , data_parts_by_info(data_parts_indexes.get<TagByInfo>())
    , data_parts_by_state_and_info(data_parts_indexes.get<TagByStateAndInfo>())
    , parts_mover(this)
{
    const auto settings = getSettings();
    allow_nullable_key = attach || settings->allow_nullable_key;