**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.
CreateMergeEntryResult createLogEntryToMergeParts(
        zkutil::ZooKeeperPtr & zookeeper,
        const DataPartsVector & parts,
        const String & merged_name,
        const UUID & merged_part_uuid,
        const MergeTreeDataPartType & merged_part_type,
        bool deduplicate,
        const Names & deduplicate_by_columns,
        ReplicatedMergeTreeLogEntryData * out_log_entry,
        int32_t log_version,
        MergeType merge_type);
    CreateMergeEntryResult createLogEntryToMutatePart(
        const IMergeTreeDataPart & part,