**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(656, MEILISEARCH_EXCEPTION) \
    M(657, UNSUPPORTED_MEILISEARCH_TYPE) \
    M(658, MEILISEARCH_MISSING_SOME_COLUMNS) \
    M(659, UNKNOWN_STATUS_OF_TRANSACTION) \
    M(660, HDFS_ERROR) \
    M(661, CANNOT_SEND_SIGNAL) \
    M(662, FS_METADATA_ERROR) \
    M(663, INCONSISTENT_METADATA_FOR_BACKUP) \
    M(664, ACCESS_STORAGE_DOESNT_ALLOW_BACKUP) \
    M(665, CANNOT_CONNECT_NATS) \
    M(666, CANNOT_USE_CACHE) \
    \
    M(999, KEEPER_EXCEPTION) \
    M(1000, POCO_EXCEPTION) \
    M(1001, STD_EXCEPTION) \
    M(1002, UNKNOWN_EXCEPTION) \
/* See END */
namespace DB
{
namespace ErrorCodes
{
#define M(VALUE, NAME) extern const ErrorCode NAME = VALUE;
    APPLY_FOR_ERROR_CODES(M)
#undef M
    constexpr ErrorCode END = 3000;
    ErrorPairHolder values[END + 1]{};
    struct ErrorCodesNames
    {
        std::string_view names[END + 1];
        ErrorCodesNames()