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'm not sure how to argue here, but half the size as default seems a bit fraught. If a user configures 4k pages, and inserts 8 byte keys, and 2k items. Are we going to end up with a bunch of leaf pages that have a single item?