Easy node startup

[dmtrskv]

One should be able to start a node providing only the chain, like this:
nild archive --chain=testnet

It requires at least some data (for each of the chains) hard-coded:

• Genesis block: version or full zero-state.
• Relay addresses.
• Archive node identities (reachable via relays).

We should also ensure that corresponding parameters don't change unexpectedly when running nild gen-configs (with an opt-out).