**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.
{
    assertNotReadonly();
    if (!is_leader)
        throw Exception("DROP PARTITION cannot be done on this replica because it is not a leader", ErrorCodes::NOT_A_LEADER);
    zkutil::ZooKeeperPtr zookeeper = getZooKeeperAndAssertNotReadonly();
    const auto * partition_ast = partition->as<ASTPartition>();
    if (partition_ast && partition_ast->all)
    {
        dropAllPartitionsImpl(zookeeper, detach, query_context);
    }
    else
    {
        String partition_id = getPartitionIDFromQuery(partition, query_context);
        auto entry = dropAllPartsInPartition(*zookeeper, partition_id, query_context, detach);
        if (entry)
        {
            waitForLogEntryToBeProcessedIfNecessary(*entry, query_context);
            cleanLastPartNode(partition_id);
        }
    }
}
void StorageReplicatedMergeTree::truncate(