**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 ASTPtr &, const StorageMetadataPtr &, ContextPtr query_context, TableExclusiveLockHolder & table_lock)
{
    table_lock.release();   
    assertNotReadonly();
    if (!is_leader)
        throw Exception("TRUNCATE cannot be done on this replica because it is not a leader", ErrorCodes::NOT_A_LEADER);
    zkutil::ZooKeeperPtr zookeeper = getZooKeeperAndAssertNotReadonly();
    dropAllPartitionsImpl(zookeeper,  false, query_context);
}
PartitionCommandsResultInfo StorageReplicatedMergeTree::attachPartition(