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. - 

{
    public:
        Transaction(MergeTreeData & data_) : data(data_) {}
        DataPartsVector commit(MergeTreeData::DataPartsLock * acquired_parts_lock = nullptr);
        void rollback();
        
        
        void rollbackPartsToTemporaryState();
        size_t size() const { return precommitted_parts.size(); }
        bool isEmpty() const { return precommitted_parts.empty(); }
        ~Transaction()
        {
            try
            {
                rollback();
            }
            catch (...)
            {
                tryLogCurrentException("~MergeTreeData::Transaction");
            }
        }
    private:
        friend class MergeTreeData;
        MergeTreeData & data;
        DataParts precommitted_parts;
        void clear() { precommitted_parts.clear(); }