Enforce EIP-1085 style for the genesis file of beacon chain

[mhchia]

What is wrong?

In #307, BeaconConfig is generated through hardcoded chain_id, network_id, and the other configs SERENITY_CONFIG.

How can it be fixed

It is possibly useful if we move the configs to a standalone genesis-beaconchain.json, in order to 1) remove hardcoded values for genesis. 2) share the config across ETH serentiy clients.

Steps

• Create the json-schema for the beacon chain genesis file, enforcing EIP-1085-like style.
• Load and validate the genesis file with the json-schema, which is implemented in Trinity, as the genesis_config.
• Add BeaconConfig.from_beaconchain_genesis_config, to load genesis_data from the genesis_config
• Generate BeaconChain with BeaconConfig.initialize_chain, which generates the chain with BeaconConfig.genesis_data

Reference

• EIP-1085
• Discussion in trinity
• EIP-1085 schema in trinity
• EIP-1085 mainnet.json in trinity

[ralexstokes]

closing due to stalness. we can revisit if needed