If the default leaf/internal page size is 4KB, then setting it to
something smaller requires setting the allocation size as well,
because page sizes must be multiples of the allocation size.
Re-work the page, allocation size checks: instead of silently modifying
them, we've set the default to 4KB, and now we complain if the chosen
page or allocation size isn't at least as large as the buffer alignment
value, as well as a multiple of the buffer alignment value.