ITS#7713 Handle bigger OS page sizes
Use DB page size = min(32k, OS pagesize). Previous limit was 8k (MDB_MINKEYS*MDB_PAGESIZE). Handle DB pagesize < OS pagesize. That's an I/O pessimization, but transactions remain atomic: Only writing the MDB_meta must be atomic, and it fits in one OS page. Don't truncate desired subpage size: Asssign it to a size_t (mv_size), not an uint16_t (mp_upper).