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. - 

new_part_info.mutation = mutation_version;
    String new_part_name = part.getNewName(new_part_info);
    ReplicatedMergeTreeLogEntryData entry;
    entry.type = LogEntry::MUTATE_PART;
    entry.source_replica = replica_name;
    entry.source_parts.push_back(part.name);
    entry.new_part_name = new_part_name;
    entry.new_part_uuid = new_part_uuid;
    entry.create_time = time(nullptr);
    entry.alter_version = alter_version;
    Coordination::Requests ops;
    Coordination::Responses responses;
    ops.emplace_back(zkutil::makeCreateRequest(
        fs::path(zookeeper_path) / "log/log-", entry.toString(),
        zkutil::CreateMode::PersistentSequential));
    ops.emplace_back(zkutil::makeSetRequest(
        fs::path(zookeeper_path) / "log", "", log_version)); 
    Coordination::Error code = zookeeper->tryMulti(ops, responses);
    if (code == Coordination::Error::ZBADVERSION)
    {
        ProfileEvents::increment(ProfileEvents::NotCreatedLogEntryForMutation);
        LOG_TRACE(log, "Log entry is not created for mutation {} because log was updated", new_part_name);
        return CreateMergeEntryResult::LogUpdated;
    }
    zkutil::KeeperMultiException::check(code, ops, responses);
    ProfileEvents::increment(ProfileEvents::CreatedLogEntryForMutation);
    LOG_TRACE(log, "Created log entry for mutation {}", new_part_name);
    return CreateMergeEntryResult::Ok;
}
void StorageReplicatedMergeTree::removePartFromZooKeeper(const String & part_name, Coordination::Requests & ops, bool has_children)