**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.
#include <Storages/MergeTree/EphemeralLockInZooKeeper.h>
#include <Storages/MergeTree/DataPartsExchange.h>
#include <Storages/MergeTree/ReplicatedMergeTreeAddress.h>
#include <Storages/MergeTree/PartMovesBetweenShardsOrchestrator.h>
#include <Storages/MergeTree/FutureMergedMutatedPart.h>
#include <Storages/MergeTree/MergeFromLogEntryTask.h>
#include <DataTypes/DataTypesNumber.h>
#include <Interpreters/Cluster.h>
#include <Interpreters/PartLog.h>
#include <Common/randomSeed.h>
#include <Common/ZooKeeper/ZooKeeper.h>
#include <Common/Throttler.h>
#include <Core/BackgroundSchedulePool.h>
#include <QueryPipeline/Pipe.h>
#include <Storages/MergeTree/BackgroundJobsAssignee.h>
namespace DB
{
/** The engine that uses the merge tree (see MergeTreeData) and is replicated through ZooKeeper.
  *
  * ZooKeeper is used for the following things:
  * - the structure of the table (/metadata, /columns)
  * - action log with data (/log/log-...,/replicas/replica_name/queue/queue-...);
  * - a replica list (/replicas), and replica activity tag (/replicas/replica_name/is_active), replica addresses (/replicas/replica_name/host);
  * - the leader replica election (/leader_election) - these are the replicas that assign merges, mutations
  *   and partition manipulations.
  *   (after ClickHouse version 20.5 we allow multiple leaders to act concurrently);
  * - a set of parts of data on each replica (/replicas/replica_name/parts);
  * - list of the last N blocks of data with checksum, for deduplication (/blocks);
  * - the list of incremental block numbers (/block_numbers) that we are about to insert,
  *   to ensure the linear order of data insertion and data merge only on the intervals in this sequence;