Reconsider page size for large objects: rather than stuffing disk pages

Reconsider page size for large objects: rather than stuffing disk pages
as full as possible, seems better to use a tuple size around BLCKSZ/4
so that less space is wasted when a LO tuple is updated.  Also, this
lets us use a logical page size that's an exact power of two, avoiding
partial-page writes when client is sending us stuff in power-of-2
buffer chunks.