Change database system to instead send back an updated version of the chain spec

[tomaka]

The chain specification can already be made quite minimal. It mostly needs to contain just the list of bootnodes, the genesis root, and a checkpoint.

The current database format contains a list of known nodes, the genesis root, a checkpoint, and a runtime code hint. Basically, almost the same things.
If the idea in paritytech/polkadot-sdk#60 (comment) goes through, then the chain spec would also contain a runtime code hint.

Therefore, instead of having a separate database format, maybe we could simply overwrite the chain specification of the chain with new information.

The main drawback is that the chain spec format should match the one of Substrate, making it more difficult to add more fields to the database later if necessary. Right now I don't see any other field that we might want to add.

[tomaka]

cc #1486