**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.
const UUID & new_part_uuid,
        Int64 mutation_version,
        int32_t alter_version,
        int32_t log_version);
    
    ConnectionTimeouts getFetchPartHTTPTimeouts(ContextPtr context);
    /** Returns an empty string if no one has a part.
      */
    String findReplicaHavingPart(const String & part_name, bool active);
    static String findReplicaHavingPart(const String & part_name, const String & zookeeper_path_, zkutil::ZooKeeper::Ptr zookeeper_);
    bool checkReplicaHavePart(const String & replica, const String & part_name);
    bool checkIfDetachedPartExists(const String & part_name);
    bool checkIfDetachedPartitionExists(const String & partition_name);
    /** Find replica having specified part or any part that covers it.
      * If active = true, consider only active replicas.
      * If found, returns replica name and set 'entry->actual_new_part_name' to name of found largest covering part.
      * If not found, returns empty string.
      */
    String findReplicaHavingCoveringPart(LogEntry & entry, bool active);
    String findReplicaHavingCoveringPart(const String & part_name, bool active, String & found_part_name);
    /** Download the specified part from the specified replica.
      * If `to_detached`, the part is placed in the `detached` directory.
      * If quorum != 0, then the node for tracking the quorum is updated.
      * Returns false if part is already fetching right now.
      */
    bool fetchPart(
        const String & part_name,
        const StorageMetadataPtr & metadata_snapshot,
        const String & replica_path,
        bool to_detached,