lib: Always checksum content in deltas

lib: Always checksum content in deltas

This is a follow up to conversation on list - in practice, if we're
backing away from summary signing, then it makes sense to remove the
special casing for checksums in deltas around summary signatures.

This is also related to the recent change to enable GPG checking for
commits in deltas - now we have a more coherent story between the
previous pull path and deltas.

I didn't do any performance checking, and while it's slightly annoying
that we're now doing sha256 on the delta content twice (once for the
part and once per object)...sha256 is pretty fast, I think most users
are I/O bound anyways, and it'd drop even farther if we started using
openssl.

Closes: #612
Approved by: jlebon