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 String & zookeeper_path_,
    const String & replica_name_,
    bool attach,
    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> settings_,
    bool has_force_restore_data_flag,
    bool allow_renaming_)
    : MergeTreeData(table_id_,
                    relative_data_path_,