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. - 

Int64 startMutation(const MutationCommands & commands, String & mutation_file_name);
    
    void waitForMutation(Int64 version, const String & file_name);
    struct CurrentlyMergingPartsTagger
    {
        FutureMergedMutatedPart future_part;
        ReservationPtr reserved_space;
        StorageMergeTree & storage;
        
        std::optional<CurrentlySubmergingEmergingTagger> tagger;
        CurrentlyMergingPartsTagger(
            FutureMergedMutatedPart & future_part_,
            size_t total_size,
            StorageMergeTree & storage_,
            const StorageMetadataPtr & metadata_snapshot,
            bool is_mutation);
        ~CurrentlyMergingPartsTagger();
    };
    using CurrentlyMergingPartsTaggerPtr = std::unique_ptr<CurrentlyMergingPartsTagger>;
    friend struct CurrentlyMergingPartsTagger;
    struct MergeMutateSelectedEntry
    {
        FutureMergedMutatedPart future_part;
        CurrentlyMergingPartsTaggerPtr tagger;
        MutationCommands commands;
        MergeMutateSelectedEntry(const FutureMergedMutatedPart & future_part_, CurrentlyMergingPartsTaggerPtr && tagger_, const MutationCommands & commands_)
            : future_part(future_part_)
            , tagger(std::move(tagger_))
            , commands(commands_)