Port AuRa engine from Rust to Go

[5chdn]

• review AuRa consensus engine in Parity & config
• port consensus engine to Go

[priom]

https://github.com/paritytech/parity-ethereum/search?q=kovan&unscoped_q=kovan
https://github.com/paritytech/parity-ethereum/blob/1a16f335fac64bbfe3e6157a053d5ca6fce3d09d/scripts/aura-test.sh
https://github.com/paritytech/parity-ethereum/blob/74ce0f738e0cd09b8422ba5d7847105654143676/json/src/spec/authority_round.rs
https://github.com/paritytech/parity-ethereum/tree/master/ethcore/res
https://github.com/paritytech/parity-ethereum/blob/master/ethcore/res/ethereum/kovan.json

[noot]

this is a preliminary list of functions that may need to be modified. however, not all of these may need to be modified. there are likely many many more places that /do/ need to be modified.

Changes to be made in aura.go (implementing Engine interface):
Author():

• Pull the author from block header (not coinbase)

VerifyHeader():

• Determine what checks need to be placed here
• Implement checks
• (???) mixDigest; i don't think parity implements mixDigest

VerifyHeaders():

• Confirm not changes needed

VerifyUncles():

• Return true always

VerifySeal():

• make sure the signature of the header is the correct elected authority based on turns

Prepare()

• Determine if no changes necessary (ie. no additional fields in header)

Finalize()

• Add block reward (5 eth)
• Add tx fees (assumption)

Seal()

• Verify no changes needed
• make sure to sign correct block

SealHash()

• Verify no changes needed

CalcDifficulty()

• Verify no changes needed (presumable a constant)

Other Functions maybe needed:

verifyCascadingFields():

• If the block is a checkpoint block, verify the signer list

snapshot()

• how often are snapshots created? -> snapshots are not necessary for Aura, as long as we store validators somewhere and they are static