ITS#7589 Fix me_nodemax and related node sizes.
Change me_nodemax to not count the mp_ptrs entry. That's mostly how it was used. Compare node sizes ">" me_nodemax instead of ">=". The ">=" was a workaround for confusing sizes with and without the mp_ptrs entry, but broke for nodes with size (old me_nodemax-1). Explicitly make me_nodemax even. An odd value could break the comparisons. It was even anyway because MDB_MINKEYS == 2.