use protobufs to encode index values

[mschoch]

While we can't use them for the index keys which we craft to get the desired sort order, we should use protobufs to encode the index values. This will make the binary serialization/deserialization less error prone, more compact, and easier to evolve over time.

[tleyden]

You might want to check out Cap'n Proto as an alternative to protobufs. I haven't used either yet, but @ajres has been talking about Cap'n Proto lately so I figured I'd mention it.

[mschoch]

Thanks, @tleyden. Speed is key in this use case, so I will definitely take a look.

[mschoch]

we now use protobufs for encoding the back index values

this was partly due to the complexity of the back index structure increasing, and also a useful way to see how we liked using protobufs here.

[mschoch]

Nothing actionable here any more.