**Describe the solution you'd like**

We already can run clickhouse-server without configured ZooKeeper (the tables will start in read-only mode)
and with configured ZooKeeper, if it starts to be inaccessible at runtime (the tables will be read-only until the session is re-established).

Let's also make it possible to start up when ZooKeeper is unavailable.

**Additional context**

If that will be very hard to implement, we can consider other options:
- lazy attaching of the tables after sever start-up;
- a maintenance mode when the tables are not attached at start-up and require manual ATTACH queries.
{
    return queue.getMutationsStatus();
}
CancellationCode StorageReplicatedMergeTree::killMutation(const String & mutation_id)
{
    assertNotReadonly();
    zkutil::ZooKeeperPtr zookeeper = getZooKeeperAndAssertNotReadonly();
    LOG_INFO(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(getStorageID(), partition_id, block_number);
    }
    return CancellationCode::CancelSent;
}
bool StorageReplicatedMergeTree::hasLightweightDeletedMask() const