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. - 

constexpr static auto DETACHED_DIR_NAME = "detached";
    
    struct DataPartStateAndInfo
    {
        DataPartState state;
        const MergeTreePartInfo & info;
    };
    
    struct DataPartStateAndPartitionID
    {
        DataPartState state;
        String partition_id;
    };
    STRONG_TYPEDEF(String, PartitionID)
    
    
    
    
    
    struct AlterConversions
    {
        
        std::unordered_map<String, String> rename_map;
        bool isColumnRenamed(const String & new_name) const { return rename_map.count(new_name) > 0; }
        String getColumnOldName(const String & new_name) const { return rename_map.at(new_name); }
    };
    struct LessDataPart
    {
        using is_transparent = void;
        bool operator()(const DataPartPtr & lhs, const MergeTreePartInfo & rhs) const { return lhs->info < rhs; }