`geth makedag <blocknumber> <path>` was creating DAGs for
`<blocknumber>/<epoch_length> + 1`, hence
it was impossible to create an epoch 0 DAG.
This fixes the calculations in `consensus/ethash/ethash.go` for
`MakeDataset` and `MakeCache`, and applies `gofmt`.
This commit adds pluggable consensus engines to go-ethereum. In short, it
introduces a generic consensus interface, and refactors the entire codebase to
use this interface.
