Implement: Availability Store

[rphmeier]

No description provided.

[rphmeier]

The guide goes into detail on pruning and pruning constraints. We shouldn't need to implement the pruning stuff at first. Most important is just that the necessary data is stored and queryable.

@montekki What's described in the guide is pretty different from the API in the existing av_store crate. Do you think it'll be possible to port over, or should it be rewritten alongside?

[montekki]

I don't think that porting is worth the effort, I say rewrite

[rphmeier]

https://github.com/paritytech/polkadot/pull/1312/files#r451847813 - The store should also support storing full PoVs of things that we've validated, and should be able to answer all QueryChunk messages out of them. The pruning rules for stored chunks and stored PoVs are different and this is described in the guide already.

Ideally it can cache the erasure-codings so that multiple subsequent QueryChunk messages don't cause erasure coding to happen over and over again. Either that or there should be as QueryAllChunks message.

Also, StorePoV should accept an optional parameter with our own validator index, which will cause it to act sometihng like a StoreChunk message as well (with the same pruning rules as PoVs, unless the candidate is backed)

[montekki]

Implemeted in #1404 . Pruning implementation is moved to #1488