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. - 

{
    return queue.getMutationsStatus();
}
CancellationCode StorageReplicatedMergeTree::killMutation(const String & mutation_id)
{
    assertNotReadonly();
    zkutil::ZooKeeperPtr zookeeper = getZooKeeper();
    LOG_TRACE(log, "Killing mutation {}", mutation_id);
    auto mutation_entry = queue.removeMutation(zookeeper, mutation_id);
    if (!mutation_entry)
        return CancellationCode::NotFound;
    
    
    
    for (const auto & pair : mutation_entry->block_numbers)
    {
        const String & partition_id = pair.first;
        Int64 block_number = pair.second;
        getContext()->getMergeList().cancelPartMutations(partition_id, block_number);
    }
    return CancellationCode::CancelSent;
}
void StorageReplicatedMergeTree::removePartsFromFilesystem(const DataPartsVector & parts)