The key and value offsets and lengths were aggressively packed into the
item structs in the segments. This saved a few bytes per item but
didn't leave any room left for expansion without growing the item. We
want to add a deletion item flag so let's just grow the item struct. It
now has room for full precision offsets and lengths that we can access
natively so we can get rid fo the packing and unpacking functions.
Signed-off-by: Zach Brown <zab@versity.com>