Implement genesis chain selection rule

[edsko]

If things work out in #805, we should be able to implement the genesis chain selection rule in much the same way that we have implemented the regular Praos/longest chain selection rule.

Ideally this is then independent of the implementation of the chain sync client (#221)

[edsko]

Note that we will probably not do this at all, and instead have genesis as a special mode in the chain sync client instead (#221). Note that this would be a significant deviation from the paper, which therefore needs a careful description and approval from the researchers (#219).

[edsko]

@dcoutts @i-o-m I've removed the "Shelley mainnet" label from all genesis related tickets. They are still marked as genesis but we will not be implementing genesis for the Shelley release.

[dnadales]

@nfrisby, is this issue still relevant?