**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 fetcher.fetchPart(
                metadata_snapshot,
                getContext(),
                part_name,
                source_replica_path,
                address.host,
                address.replication_port,
                timeouts,
                credentials->getUser(),
                credentials->getPassword(),
                interserver_scheme,
                replicated_fetches_throttler,
                to_detached,
                "",
                &tagger_ptr,
                try_fetch_shared);
        };
    }
    try
    {
        part = get_part();
        auto builder = part->data_part_storage->getBuilder();
        if (!to_detached)
        {
            Transaction transaction(*this, NO_TRANSACTION_RAW);
            renameTempPartAndReplace(part, transaction, builder);
            replaced_parts = checkPartChecksumsAndCommit(transaction, part, hardlinked_files);
            /** If a quorum is tracked for this part, you must update it.
              * If you do not have time, in case of losing the session, when you restart the server - see the `ReplicatedMergeTreeRestartingThread::updateQuorumIfWeHavePart` method.
              */