Organize consensus code for shards

[adlrocha]

We currently support exclusively delegated consensus in shards, so much of the code in https://github.com/filecoin-project/eudico/blob/sharding/chain/sharding/genesis.go is exclusive for delegated consensus.

We should:

• Extract this code so we have implementation for every consensus we will support in shards.
• Fix hard-coded variables and get them from the actor initialization.
• In

  eudico/chain/sharding/mine.go

  Line 14 in 3b88d82

  func (sh *Shard) mineDelegated(ctx context.Context) error {

  there is a lot of duplicate code from DelegatedMinerCmd. Consider extracting this into a package that both can use (I think it make sense adding it to the consensus package).