cmd/node: configuration changes for genesis id

[dshulyak]

part of: spacemeshos/pm#135
depends on: #3505

• add a genesis section into config
• for this issue genesis section needs to include: genesis time and genesis extra data (other fields can be moved into this config later)
  • genesis time is a string with time encoded according to RFC3339
  • genesis extra data is a string with a max size of 255 chars. defaults to mainnet
• when node is started check if genesis section is already stored in the data directory (example: /data/genesis.conf)
  • if it is not stored, encode genesis section with json and persist it on disk
  • if stored - decode object with json into another instance of genesis config, compute diff using https://github.com/google/go-cmp/ and print fatal error to the operator with a diff and a path to persisted genesis conf
• compute genesis id as a hash from genesis time and genesis extra data
• golden atx should be set to genesis id