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. - 

{
    
    String replica = findReplicaHavingCoveringPart(entry, true);
    const auto storage_settings_ptr = getSettings();
    auto metadata_snapshot = getInMemoryMetadataPtr();
    if (storage_settings_ptr->replicated_max_parallel_fetches &&
        total_fetches >= storage_settings_ptr->replicated_max_parallel_fetches)
        throw Exception(ErrorCodes::TOO_MANY_FETCHES, "Too many total fetches from replicas, maximum: {} ",
            storage_settings_ptr->replicated_max_parallel_fetches.toString());
    ++total_fetches;
    SCOPE_EXIT({--total_fetches;});
    if (storage_settings_ptr->replicated_max_parallel_fetches_for_table
        && current_table_fetches >= storage_settings_ptr->replicated_max_parallel_fetches_for_table)
        throw Exception(ErrorCodes::TOO_MANY_FETCHES, "Too many fetches from replicas for table, maximum: {}",
            storage_settings_ptr->replicated_max_parallel_fetches_for_table.toString());
    ++current_table_fetches;
    SCOPE_EXIT({--current_table_fetches;});
    try
    {
        if (replica.empty())
        {
            /** If a part is to be written with a quorum and the quorum is not reached yet,