**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.
std::optional<ReplicatedMergeTreePartHeader> desired_part_header;
            if (!part_znode.empty())
            {
                desired_part_header = ReplicatedMergeTreePartHeader::fromString(part_znode);
            }
            else
            {
                String columns_str;
                String checksums_str;
                if (zookeeper->tryGet(fs::path(part_path) / "columns", columns_str) &&
                    zookeeper->tryGet(fs::path(part_path) / "checksums", checksums_str))
                {
                    desired_part_header = ReplicatedMergeTreePartHeader::fromColumnsAndChecksumsZNodes(columns_str, checksums_str);
                }
                else
                {
                    LOG_INFO(log, "Not checking checksums of part {} with replica {} because part was removed from ZooKeeper", part_name, source_replica_path);
                }
            }
            
            
            
            if (desired_part_header
                && source_part_header.getColumnsHash() == desired_part_header->getColumnsHash()
                && source_part_header.getChecksums() == desired_part_header->getChecksums())
            {
                LOG_TRACE(log, "Found local part {} with the same checksums and columns hash as {}", source_part->name, part_name);
                part_to_clone = source_part;
            }
        }