**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(626, CANNOT_SKIP_UNKNOWN_FIELD) \
    M(627, BACKUP_ENGINE_NOT_FOUND) \
    M(628, OFFSET_FETCH_WITHOUT_ORDER_BY) \
    M(629, HTTP_RANGE_NOT_SATISFIABLE) \
    M(630, HAVE_DEPENDENT_OBJECTS) \
    M(631, UNKNOWN_FILE_SIZE) \
    M(632, UNEXPECTED_DATA_AFTER_PARSED_VALUE) \
    M(633, QUERY_IS_NOT_SUPPORTED_IN_WINDOW_VIEW) \
    M(634, MONGODB_ERROR) \
    M(635, CANNOT_POLL) \
    M(636, CANNOT_EXTRACT_TABLE_STRUCTURE) \
    M(637, INVALID_TABLE_OVERRIDE) \
    M(638, SNAPPY_UNCOMPRESS_FAILED) \
    M(639, SNAPPY_COMPRESS_FAILED) \
    M(640, NO_HIVEMETASTORE) \
    M(641, CANNOT_APPEND_TO_FILE) \
    M(642, CANNOT_PACK_ARCHIVE) \
    M(643, CANNOT_UNPACK_ARCHIVE) \
    M(644, REMOTE_FS_OBJECT_CACHE_ERROR) \
    M(645, NUMBER_OF_DIMENSIONS_MISMATCHED) \
    M(646, CANNOT_BACKUP_DATABASE) \
    M(647, CANNOT_BACKUP_TABLE) \
    M(648, WRONG_DDL_RENAMING_SETTINGS) \
    M(649, INVALID_TRANSACTION) \
    M(650, SERIALIZATION_ERROR) \
    M(651, CAPN_PROTO_BAD_TYPE) \
    M(652, ONLY_NULLS_WHILE_READING_SCHEMA) \
    M(653, CANNOT_PARSE_BACKUP_SETTINGS) \
    M(654, WRONG_BACKUP_SETTINGS) \
    M(655, FAILED_TO_SYNC_BACKUP_OR_RESTORE) \