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. - 

{
    auto & future_part = merge_mutate_entry.future_part;
    auto table_id = getStorageID();
    auto merge_list_entry = getContext()->getMergeList().insert(table_id.database_name, table_id.table_name, future_part);
    Stopwatch stopwatch;
    MutableDataPartPtr new_part;
    auto write_part_log = [&] (const ExecutionStatus & execution_status)
    {
        writePartLog(
            PartLogElement::MUTATE_PART,
            execution_status,
            stopwatch.elapsed(),
            future_part.name,
            new_part,
            future_part.parts,
            merge_list_entry.get());
    };
    try
    {
        new_part = merger_mutator.mutatePartToTemporaryPart(
            future_part, metadata_snapshot, merge_mutate_entry.commands, *(merge_list_entry),
            time(nullptr), getContext(), merge_mutate_entry.tagger->reserved_space, table_lock_holder);
        renameTempPartAndReplace(new_part);
        updateMutationEntriesErrors(future_part, true, "");
        write_part_log({});
    }
    catch (...)
    {
        updateMutationEntriesErrors(future_part, false, getCurrentExceptionMessage(false));
        write_part_log(ExecutionStatus::fromCurrentException());