**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.
{
    String part_path = fs::path(replica_path) / "parts" / part_name;
    if (has_children)
    {
        ops.emplace_back(zkutil::makeRemoveRequest(fs::path(part_path) / "checksums", -1));
        ops.emplace_back(zkutil::makeRemoveRequest(fs::path(part_path) / "columns", -1));
    }
    ops.emplace_back(zkutil::makeRemoveRequest(part_path, -1));
}
void StorageReplicatedMergeTree::removePartFromZooKeeper(const String & part_name)
{
    auto zookeeper = getZooKeeper();
    String part_path = fs::path(replica_path) / "parts" / part_name;
    Coordination::Stat stat;
    
    if (!zookeeper->exists(part_path, &stat))
        return;
    Coordination::Requests ops;
    removePartFromZooKeeper(part_name, ops, stat.numChildren > 0);
    zookeeper->multi(ops);
}
void StorageReplicatedMergeTree::removePartAndEnqueueFetch(const String & part_name)