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. - 

bool supportsParallelInsert() const override { return true; }
    bool supportsReplication() const override { return true; }
    bool supportsDeduplication() const override { return true; }
    Pipe read(
        const Names & column_names,
        const StorageMetadataPtr & ,
        SelectQueryInfo & query_info,
        ContextPtr context,
        QueryProcessingStage::Enum processed_stage,
        size_t max_block_size,
        unsigned num_streams) override;
    void read(
        QueryPlan & query_plan,
        const Names & column_names,
        const StorageMetadataPtr & ,
        SelectQueryInfo & query_info,
        ContextPtr context,
        QueryProcessingStage::Enum processed_stage,
        size_t max_block_size,
        unsigned num_streams) override;
    std::optional<UInt64> totalRows(const Settings & settings) const override;
    std::optional<UInt64> totalRowsByPartitionPredicate(const SelectQueryInfo & query_info, ContextPtr context) const override;
    std::optional<UInt64> totalBytes(const Settings & settings) const override;
    BlockOutputStreamPtr write(const ASTPtr & query, const StorageMetadataPtr & , ContextPtr context) override;