Store: only store header + DA header

[liamsi]

Summary

Tendermint currently stores the block including all data in its own block store (interface).

For LazyLedger only storing the header + DA header is sufficient. The block data, or rather more often only a portion of the data, will be stored on ipfs. See #163, #178

Problem Definition

Storing the data additionally in tendermint's store will be redundant as it is already stored (and pinned) on ipfs. Also, some nodes won't even have the whole block data.

Proposal

Update the block store accordingly. The design should be fleshed out upfront.

This depends on #178 and a corresponding "read equivalent" where the block data can be sampled or be fully reconstructed from the network using ipfs (or, more generally speaking, a DHT + some IPLD block-exchange protocol, e.g. graph-sync later down the road).

Note that fleshing out the design and starting the implementation can still be done independently. Only full integration of the feature and replacing the existing storage mechanism depends on adding the possibility to read/write to ipld.

[liamsi]

Closing this for now as we decided to treat tendermint more as blackbox until we have our devnet out of the door. We can later optimize to only store the block data once (only on the Celestia nodes) but for now we won't work on that.

Let's not delete the branch yet though.