Support sparkey_logiter_valuecmp and API changes.

[admtnnr]

Depends on the merge of https://github.com/spotify/sparkey/tree/krka/experimental_diff.

[spkrka]

Not sure it's a good to depend on this yet. It's not set in stone if it will ever be merged, or exactly how it will work. That's why it's called experimental :)

[admtnnr]

No worries, I'll leave it open until it either gets merged or the branch gets deleted, in which case I'll either merge or rebase out the first commit. Sparkey doesn't get too many commits so I figured I'd go ahead and knock it out in the off chance it makes it to master. :)

Though, I did find it odd that the valuecmp function takes two log_readers and the keycmp only takes one (assumes it's shared between the iterators). ;)