**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.
else if (code == Coordination::Error::ZBADVERSION)
    {
        ProfileEvents::increment(ProfileEvents::NotCreatedLogEntryForMerge);
        LOG_TRACE(log, "Log entry is not created for merge {} because log was updated", merged_name);
        return CreateMergeEntryResult::LogUpdated;
    }
    else
    {
        zkutil::KeeperMultiException::check(code, ops, responses);
    }
    if (out_log_entry)
        *out_log_entry = entry;
    return CreateMergeEntryResult::Ok;
}
StorageReplicatedMergeTree::CreateMergeEntryResult StorageReplicatedMergeTree::createLogEntryToMutatePart(
    const IMergeTreeDataPart & part, const UUID & new_part_uuid, Int64 mutation_version, int32_t alter_version, int32_t log_version)
{
    auto zookeeper = getZooKeeper();
    
    if (!zookeeper->exists(fs::path(replica_path) / "parts" / part.name))
    {
        if (part.modification_time + MAX_AGE_OF_LOCAL_PART_THAT_WASNT_ADDED_TO_ZOOKEEPER < time(nullptr))
        {
            LOG_WARNING(log, "Part {} (that was selected for mutation) with age {} seconds exists locally but not in ZooKeeper."
                " Won't mutate that part and will check it.", part.name, (time(nullptr) - part.modification_time));
            enqueuePartForCheck(part.name);
        }
        return CreateMergeEntryResult::MissingPart;
    }
    MergeTreePartInfo new_part_info = part.info;