Tables' Primary Keys Lazily Loaded in Memory with New MergeTree Setting

The tables' primary keys will be loaded in memory lazily on first access. This is controlled by the new MergeTree setting primary_key_lazy_load, which is on by default. This provides several advantages: - it will not be loaded for tables that are not used; - if there is not enough memory, an exception will be thrown on first use instead of at server startup. This provides several disadvantages: - the latency of loading the primary key will be paid on the first query rather than before accepting connections; this theoretically may introduce a thundering-herd problem. This closes #11188. #60093 (Alexey Milovidov).