chain: support checkpoints in the genesis state

[erwanor]

Closes #1809, and prepares #3004 since we will be able to match the supplied genesis state to decide whether we want to run chain initialization logic e.g. setting the genesis base rate, or fast-forward to committing the initial height block.

This PR turns genesis::AppState into an enum:

pub enum AppState {
    Content(/* a full genesis configuration */),
    Checkpoint(/* a free-form byte string e.g. an apphash, a commit hash */)
}

and percolate those changes across the codebase.