**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.
fetched = fetchPart(part, metadata_snapshot, best_replica_path, true, 0, zookeeper,  false);
            }
            catch (const DB::Exception & e)
            {
                if (e.code() != ErrorCodes::RECEIVED_ERROR_FROM_REMOTE_IO_SERVER && e.code() != ErrorCodes::RECEIVED_ERROR_TOO_MANY_REQUESTS
                    && e.code() != ErrorCodes::CANNOT_READ_ALL_DATA)
                    throw;
                LOG_INFO(log, fmt::runtime(e.displayText()));
            }
            if (!fetched)
                missing_parts.push_back(part);
        }
        ++try_no;
    } while (!missing_parts.empty());
}
void StorageReplicatedMergeTree::mutate(const MutationCommands & commands, ContextPtr query_context)
{