[Meta issue] Sonic v2 planning

[RemiBardon]

Following up on #372, I’ll change the planning I had in mind for Sonic v2.0.0.

The main mindset change is the order of things. I was planning on fixing things and making gradual non-breaking changes first, then making substantial (and breaking) changes, but @baptistejamin is right we should revert this. We will now make the breaking release first (one requiring a full re-ingestion) but make Sonic store the original documents so other breaking changes can become simpler migrations.

I’ll merge the few things I was working on that can easily be merged, then create a v2 branch where breaking features will be introduced.

I will now proceed replicating every step introduced in #372.

If you have any objection, here is the place to discuss about it.

[baptistejamin]

The decision will be to valerian (off until last week), and I won't interfere with it, but still research and testing can help with your planning with v2. Storage improvements will be for shure 😎 needed, especially to tackle problems with the 1k object per term limit, and backingestion speed.