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. - 

StorageMetadataPtr metadata_snapshot = getInMemoryMetadataPtr();
    MutableDataPartPtr new_part;
    Transaction transaction(*this);
    FutureMergedMutatedPart future_mutated_part;
    future_mutated_part.name = entry.new_part_name;
    future_mutated_part.uuid = entry.new_part_uuid;
    future_mutated_part.parts.push_back(source_part);
    future_mutated_part.part_info = new_part_info;
    future_mutated_part.updatePath(*this, reserved_space);
    future_mutated_part.type = source_part->getType();
    auto table_id = getStorageID();
    MergeList::EntryPtr merge_entry = getContext()->getMergeList().insert(
        table_id.database_name, table_id.table_name, future_mutated_part);
    Stopwatch stopwatch;
    auto write_part_log = [&] (const ExecutionStatus & execution_status)
    {
        writePartLog(
            PartLogElement::MUTATE_PART, execution_status, stopwatch.elapsed(),
            entry.new_part_name, new_part, future_mutated_part.parts, merge_entry.get());
    };
    try
    {
        new_part = merger_mutator.mutatePartToTemporaryPart(
            future_mutated_part, metadata_snapshot, commands, *merge_entry,
            entry.create_time, getContext(), reserved_space, table_lock);
        renameTempPartAndReplace(new_part, nullptr, &transaction);
        try
        {
            checkPartChecksumsAndCommit(transaction, new_part);
        }