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).