**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.
M(536, CANNOT_RESTORE_FROM_FIELD_DUMP) \
    M(537, ILLEGAL_MYSQL_VARIABLE) \
    M(538, MYSQL_SYNTAX_ERROR) \
    M(539, CANNOT_BIND_RABBITMQ_EXCHANGE) \
    M(540, CANNOT_DECLARE_RABBITMQ_EXCHANGE) \
    M(541, CANNOT_CREATE_RABBITMQ_QUEUE_BINDING) \
    M(542, CANNOT_REMOVE_RABBITMQ_EXCHANGE) \
    M(543, UNKNOWN_MYSQL_DATATYPES_SUPPORT_LEVEL) \
    M(544, ROW_AND_ROWS_TOGETHER) \
    M(545, FIRST_AND_NEXT_TOGETHER) \
    M(546, NO_ROW_DELIMITER) \
    M(547, INVALID_RAID_TYPE) \
    M(548, UNKNOWN_VOLUME) \
    M(549, DATA_TYPE_CANNOT_BE_USED_IN_KEY) \
    M(550, CONDITIONAL_TREE_PARENT_NOT_FOUND) \
    M(551, ILLEGAL_PROJECTION_MANIPULATOR) \
    M(552, UNRECOGNIZED_ARGUMENTS) \
    M(553, LZMA_STREAM_ENCODER_FAILED) \
    M(554, LZMA_STREAM_DECODER_FAILED) \
    M(555, ROCKSDB_ERROR) \
    M(556, SYNC_MYSQL_USER_ACCESS_ERROR)\
    M(557, UNKNOWN_UNION) \
    M(558, EXPECTED_ALL_OR_DISTINCT) \
    M(559, INVALID_GRPC_QUERY_INFO) \
    M(560, ZSTD_ENCODER_FAILED) \
    M(561, ZSTD_DECODER_FAILED) \
    M(562, TLD_LIST_NOT_FOUND) \
    M(563, CANNOT_READ_MAP_FROM_TEXT) \
    M(564, INTERSERVER_SCHEME_DOESNT_MATCH) \
    M(565, TOO_MANY_PARTITIONS) \