**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.
{
    const auto storage_settings_ptr = getSettings();
    return storage_settings_ptr->index_granularity_bytes != 0 &&
        (storage_settings_ptr->enable_mixed_granularity_parts ||
            (!has_non_adaptive_index_granularity_parts && !other_replicas_fixed_granularity));
}
MutationCommands StorageReplicatedMergeTree::getFirstAlterMutationCommandsForPart(const DataPartPtr & part) const
{
    return queue.getFirstAlterMutationCommandsForPart(part);
}
void StorageReplicatedMergeTree::startBackgroundMovesIfNeeded()
{
    if (areBackgroundMovesNeeded())
        background_moves_assignee.start();
}
std::unique_ptr<MergeTreeSettings> StorageReplicatedMergeTree::getDefaultSettings() const
{
    return std::make_unique<MergeTreeSettings>(getContext()->getReplicatedMergeTreeSettings());
}
String StorageReplicatedMergeTree::getTableSharedID() const
{
    return toString(table_shared_id);
}
void StorageReplicatedMergeTree::createTableSharedID()