**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.
assignee.scheduleFetchTask(std::make_shared<ExecutableLambdaAdapter>(
            [this, selected_entry] () mutable
            {
                return processQueueEntry(selected_entry);
            }, common_assignee_trigger, getStorageID()));
        return true;
    }
    else if (job_type == LogEntry::MERGE_PARTS)
    {
        auto task = std::make_shared<MergeFromLogEntryTask>(selected_entry, *this, common_assignee_trigger);
        assignee.scheduleMergeMutateTask(task);
        return true;
    }
    else if (job_type == LogEntry::MUTATE_PART)
    {
        auto task = std::make_shared<MutateFromLogEntryTask>(selected_entry, *this, common_assignee_trigger);
        assignee.scheduleMergeMutateTask(task);
        return true;
    }
    else
    {
        assignee.scheduleCommonTask(std::make_shared<ExecutableLambdaAdapter>(
            [this, selected_entry] () mutable
            {
                return processQueueEntry(selected_entry);
            }, common_assignee_trigger, getStorageID()),  true);
        return true;
    }
}
bool StorageReplicatedMergeTree::canExecuteFetch(const ReplicatedMergeTreeLogEntry & entry, String & disable_reason) const