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 zookeeper = getZooKeeper();
    String metadata_str = zookeeper->get(zookeeper_path + "/metadata");
    auto metadata_from_zk = ReplicatedMergeTreeTableMetadata::parse(metadata_str);
    return metadata_from_zk.index_granularity_bytes == 0;
}
void StorageReplicatedMergeTree::waitMutationToFinishOnReplicas(
    const Strings & replicas, const String & mutation_id) const
{
    if (replicas.empty())
        return;
    std::set<String> inactive_replicas;
    for (const String & replica : replicas)
    {
        LOG_DEBUG(log, "Waiting for {} to apply mutation {}", replica, mutation_id);
        zkutil::EventPtr wait_event = std::make_shared<Poco::Event>();
        while (!partial_shutdown_called)
        {