Create engine for AuRa integration in Geth

[5chdn]

• review Clique engine implementation in Geth
• review Rinkeby specification in Geth
• prepare AuRa engine in Geth

[noot]

https://github.com/ethereum/go-ethereum/blob/master/consensus/clique/clique.go
this is the clique package in geth

https://gowalker.org/github.com/ethereum/go-ethereum/consensus
this is the Engine interface we need to implement

[ansermino]

Questions from Aura Spec (https://wiki.parity.io/Aura):

• Confirm block time (15 seconds currently)
  4 seconds https://github.com/goerli/testnet/blob/master/parity/goerli.json#L7
• clarify how validators are added
• What is C[K..]
• How are empty block producers rewarded?
• Are empty step messages enabled in Parity? Lower priority task (empty block instead)?
  no, that's disabled, low priority task
• What are the parity maximumEmptyStepTransition?
  not to be implemented or low priority task (see above)
• What is the empty step message format? (Parity)
  not to be implemented or low priority task (see above)
• Does the hash in the empty step messages change for consecutive empty steps?
  not to be implemented or low priority task (see above)
• What is a blockchain address (ie --engine-signer VALIDATOR-ADDRESS)?
  that's the public address of the authority node allowed to seal blocks
• What is the role of --force-sealing with empty step messages?