**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(504, FILE_ALREADY_EXISTS) \
    M(505, CANNOT_DELETE_DIRECTORY) \
    M(506, UNEXPECTED_ERROR_CODE) \
    M(507, UNABLE_TO_SKIP_UNUSED_SHARDS) \
    M(508, UNKNOWN_ACCESS_TYPE) \
    M(509, INVALID_GRANT) \
    M(510, CACHE_DICTIONARY_UPDATE_FAIL) \
    M(511, UNKNOWN_ROLE) \
    M(512, SET_NON_GRANTED_ROLE) \
    M(513, UNKNOWN_PART_TYPE) \
    M(514, ACCESS_STORAGE_FOR_INSERTION_NOT_FOUND) \
    M(515, INCORRECT_ACCESS_ENTITY_DEFINITION) \
    M(516, AUTHENTICATION_FAILED) \
    M(517, CANNOT_ASSIGN_ALTER) \
    M(518, CANNOT_COMMIT_OFFSET) \
    M(519, NO_REMOTE_SHARD_AVAILABLE) \
    M(520, CANNOT_DETACH_DICTIONARY_AS_TABLE) \
    M(521, ATOMIC_RENAME_FAIL) \
    M(523, UNKNOWN_ROW_POLICY) \
    M(524, ALTER_OF_COLUMN_IS_FORBIDDEN) \
    M(525, INCORRECT_DISK_INDEX) \
    M(527, NO_SUITABLE_FUNCTION_IMPLEMENTATION) \
    M(528, CASSANDRA_INTERNAL_ERROR) \
    M(529, NOT_A_LEADER) \
    M(530, CANNOT_CONNECT_RABBITMQ) \
    M(531, CANNOT_FSTAT) \
    M(532, LDAP_ERROR) \
    M(533, INCONSISTENT_RESERVATIONS) \
    M(534, NO_RESERVATIONS_PROVIDED) \
    M(535, UNKNOWN_RAID_TYPE) \