Spec out FastSync and StateSync

[liamsi]

Will add more details shortly

[liamsi]

I think, it makes sense to at least consider writing a custom block-sync (aka fast-sync) reactor using libp2p instead of modifying the current blockchain reactors:
https://github.com/lazyledger/lazyledger-core/tree/master/blockchain

Alternatively, and the first thing we should investigate thoroughly: we could make the blockchain reactor load the block data from the IPFS data store instead of the tendermint store - which will only store headers moving forward: #182.
We would basically change this line and in the blockchain reactor to load the whole block (locally) from the IPFS datastore: https://github.com/lazyledger/lazyledger-core/blob/c48bd16994a9fb707c9e15bc76070c9a4eeb29a1/blockchain/v0/reactor.go#L181

We'd have to make sure, that this works with the different kinds of validator nodes though. DAS light validator nodes would only care about the headers + DAS sampling for instance so they would not run the blockchain reactor but would still run the consensus reactor.

Given that the blockchain reactor looks kinda horrible and the v2 is still not stable (ref: #309) and also not really straightforward to understand, we should probably write our own block-sync protocol in the long run. We should first think through if we can leverage the existing one first though as it is used in production for several years now.

[liamsi]

ref: #191

[liamsi]

Closing this for now. We will use tendermint more as a black box. in case we later merge core and celestia node into one node repo, this becomes relevant again.