Rename consensus engine from dummy to something else

[darioush]

The name is a reference to "not implementing geth's consensus", but this is confusing to those who don't have the background on the name.
Since it's a bit odd to call production code dummy, let's rename it after finishing the geth update

[ceyonur]

For some context: https://github.com/ava-labs/coreth/blob/master/consensus/dummy/README.md#L29

We can even consider removing that single dummy package and just use plain consensus pkg. If we prefer to keep, I think we can go with either something with verifier or avalanche/ava.