Separate the btree maximum key/value sizes from the underlying page size

Separate the btree maximum key/value sizes from the underlying page size, reference #1282.

Yes, we likely will, but I'd argue that 4KB pages with 2KB items is a pretty bad idea.

There are reasons to make this smaller (so we're less likely to see single items on a page), but there are also reasons to make it larger (so we're less likely to see overflow items where it doesn't matter, for example, 32KB doesn't affect the cache much, why make it an overflow item?)

I could see making this a constant, too: we store items "in the page" until we get to 32KB or 64KB, that is, we don't make anything under 32/64KB an overflow item.