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.

I noticed the original value didn't take into account the addresses we're storing with each key, so this change tries to get us to roughly 8 keys per page. That said, I've got no evidence 8 is right, we kind of just picked a number awhile back and stuck with it.