**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.
{
    if (action_type == ActionLocks::PartsMerge)
        return merger_mutator.merges_blocker.cancel();
    if (action_type == ActionLocks::PartsTTLMerge)
        return merger_mutator.ttl_merges_blocker.cancel();
    if (action_type == ActionLocks::PartsFetch)
        return fetcher.blocker.cancel();
    if (action_type == ActionLocks::PartsSend)
    {
        auto data_parts_exchange_ptr = std::atomic_load(&data_parts_exchange_endpoint);
        return data_parts_exchange_ptr ? data_parts_exchange_ptr->blocker.cancel() : ActionLock();
    }
    if (action_type == ActionLocks::ReplicationQueue)
        return queue.actions_blocker.cancel();
    if (action_type == ActionLocks::PartsMove)
        return parts_mover.moves_blocker.cancel();
    return {};
}
void StorageReplicatedMergeTree::onActionLockRemove(StorageActionBlockType action_type)
{
    if (action_type == ActionLocks::PartsMerge || action_type == ActionLocks::PartsTTLMerge
        || action_type == ActionLocks::PartsFetch || action_type == ActionLocks::PartsSend
        || action_type == ActionLocks::ReplicationQueue)
        background_operations_assignee.trigger();
    else if (action_type == ActionLocks::PartsMove)
        background_moves_assignee.trigger();
}
bool StorageReplicatedMergeTree::waitForShrinkingQueueSize(size_t queue_size, UInt64 max_wait_milliseconds)