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. - 

Strings getDataPaths() const override;
    using PathsWithDisks = std::vector<PathWithDisk>;
    PathsWithDisks getRelativeDataPathsWithDisks() const;
    
    ReservationPtr reserveSpace(UInt64 expected_size) const;
    
    static ReservationPtr reserveSpace(UInt64 expected_size, SpacePtr space);
    static ReservationPtr tryReserveSpace(UInt64 expected_size, SpacePtr space);
    
    ReservationPtr reserveSpacePreferringTTLRules(
        const StorageMetadataPtr & metadata_snapshot,
        UInt64 expected_size,
        const IMergeTreeDataPart::TTLInfos & ttl_infos,
        time_t time_of_move,
        size_t min_volume_index = 0,
        bool is_insert = false,
        DiskPtr selected_disk = nullptr) const;
    ReservationPtr tryReserveSpacePreferringTTLRules(
        const StorageMetadataPtr & metadata_snapshot,
        UInt64 expected_size,
        const IMergeTreeDataPart::TTLInfos & ttl_infos,
        time_t time_of_move,
        size_t min_volume_index = 0,
        bool is_insert = false,
        DiskPtr selected_disk = nullptr) const;
    
    
    
    ReservationPtr balancedReservation(