Set the id2entry database pagesize to BDB_ID2ENTRY_PAGESIZE to avoid

Set the id2entry database pagesize to BDB_ID2ENTRY_PAGESIZE to avoid
overflow/fragmentation. (This is now 16K vs default 4K.) It turns out
that the entries' on-disk format is quite space-inefficient, storing
4 bytes per pointer or integer when typically >50% of those bytes are
zero. Oh well. It's about a 2:1 space increase over ldbm now, vs 4:1
before when all the entries were overflowing the 4K pages.