**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(Bool, remove_rolled_back_parts_immediately, 1, "Setting for an incomplete experimental feature.", 0) \
    \
    /** Inserts settings. */ \
    M(UInt64, parts_to_delay_insert, 150, "If table contains at least that many active parts in single partition, artificially slow down insert into table.", 0) \
    M(UInt64, inactive_parts_to_delay_insert, 0, "If table contains at least that many inactive parts in single partition, artificially slow down insert into table.", 0) \
    M(UInt64, parts_to_throw_insert, 300, "If more than this number active parts in single partition, throw 'Too many parts ...' exception.", 0) \
    M(UInt64, inactive_parts_to_throw_insert, 0, "If more than this number inactive parts in single partition, throw 'Too many inactive parts ...' exception.", 0) \
    M(UInt64, max_delay_to_insert, 1, "Max delay of inserting data into MergeTree table in seconds, if there are a lot of unmerged parts in single partition.", 0) \
    M(UInt64, max_parts_in_total, 100000, "If more than this number active parts in all partitions in total, throw 'Too many parts ...' exception.", 0) \
    \
    /** Replication settings. */ \
    M(UInt64, replicated_deduplication_window, 100, "How many last blocks of hashes should be kept in ZooKeeper (old blocks will be deleted).", 0) \
    M(UInt64, replicated_deduplication_window_seconds, 7 * 24 * 60 * 60 /* one week */, "Similar to \"replicated_deduplication_window\", but determines old blocks by their lifetime. Hash of an inserted block will be deleted (and the block will not be deduplicated after) if it outside of one \"window\". You can set very big replicated_deduplication_window to avoid duplicating INSERTs during that period of time.", 0) \
    M(UInt64, max_replicated_logs_to_keep, 1000, "How many records may be in log, if there is inactive replica. Inactive replica becomes lost when when this number exceed.", 0) \
    M(UInt64, min_replicated_logs_to_keep, 10, "Keep about this number of last records in ZooKeeper log, even if they are obsolete. It doesn't affect work of tables: used only to diagnose ZooKeeper log before cleaning.", 0) \
    M(Seconds, prefer_fetch_merged_part_time_threshold, 3600, "If time passed after replication log entry creation exceeds this threshold and sum size of parts is greater than \"prefer_fetch_merged_part_size_threshold\", prefer fetching merged part from replica instead of doing merge locally. To speed up very long merges.", 0) \
    M(UInt64, prefer_fetch_merged_part_size_threshold, 10ULL * 1024 * 1024 * 1024, "If sum size of parts exceeds this threshold and time passed after replication log entry creation is greater than \"prefer_fetch_merged_part_time_threshold\", prefer fetching merged part from replica instead of doing merge locally. To speed up very long merges.", 0) \
    M(Seconds, execute_merges_on_single_replica_time_threshold, 0, "When greater than zero only a single replica starts the merge immediately, others wait up to that amount of time to download the result instead of doing merges locally. If the chosen replica doesn't finish the merge during that amount of time, fallback to standard behavior happens.", 0) \
    M(Seconds, remote_fs_execute_merges_on_single_replica_time_threshold, 3 * 60 * 60, "When greater than zero only a single replica starts the merge immediately if merged part on shared storage and 'allow_remote_fs_zero_copy_replication' is enabled.", 0) \
    M(Seconds, try_fetch_recompressed_part_timeout, 7200, "Recompression works slow in most cases, so we don't start merge with recompression until this timeout and trying to fetch recompressed part from replica which assigned this merge with recompression.", 0) \
    M(Bool, always_fetch_merged_part, false, "If true, replica never merge parts and always download merged parts from other replicas.", 0) \
    M(UInt64, max_suspicious_broken_parts, 10, "Max broken parts, if more - deny automatic deletion.", 0) \
    M(UInt64, max_suspicious_broken_parts_bytes, 1ULL * 1024 * 1024 * 1024, "Max size of all broken parts, if more - deny automatic deletion.", 0) \
    M(UInt64, max_files_to_modify_in_alter_columns, 75, "Not apply ALTER if number of files for modification(deletion, addition) more than this.", 0) \
    M(UInt64, max_files_to_remove_in_alter_columns, 50, "Not apply ALTER, if number of files for deletion more than this.", 0) \
    M(Float, replicated_max_ratio_of_wrong_parts, 0.5, "If ratio of wrong parts to total number of parts is less than this - allow to start.", 0) \
    M(UInt64, replicated_max_parallel_fetches_for_host, DEFAULT_COUNT_OF_HTTP_CONNECTIONS_PER_ENDPOINT, "Limit parallel fetches from endpoint (actually pool size).", 0) \
    M(Seconds, replicated_fetches_http_connection_timeout, 0, "HTTP connection timeout for part fetch requests. Inherited from default profile `http_connection_timeout` if not set explicitly.", 0) \
    M(Seconds, replicated_fetches_http_send_timeout, 0, "HTTP send timeout for part fetch requests. Inherited from default profile `http_send_timeout` if not set explicitly.", 0) \
    M(Seconds, replicated_fetches_http_receive_timeout, 0, "HTTP receive timeout for fetch part requests. Inherited from default profile `http_receive_timeout` if not set explicitly.", 0) \