2/MVDS: Change `message_id`s to a multihash

[decanus]

Problem

We currently do not use multihash meaning we are bound to a single hashing algorithm.

Solution

The message_ids are described to be multihashes.

[oskarth]

To deal with fixed length indexing problem in e.g. Go, perhaps padding to a fixed length and using that would work? I'm sure a lot of codebases using multihashes also use indexing.

[decanus]

@oskarth should work, because we will know what the length of our hashes has to be.

[oskarth]

@decanus A good follow up issue on this would be to ensure there's a good story for compatibility and upgrades. That issue would then be a blocker for this one.

Removing from milestone, if you disagree we can talk about it

[decanus]

@oskarth, I agree. We should discuss compatibility on top of MVDS more and make this a part of it.

[oskarth]

Ping re compatibility discussion issue

[jimstir]

Closing, 2/MVDS is in stable status.