make `String` methods for logged dPoS types

[aupiff]

All logged dPoS types should have pretty printing String() methods to make the logs readable.

[gakonst]

All methods generated by protoc have this https://github.com/loomnetwork/go-loom/blob/master/builtin/types/dposv2/dposv2.pb.go#L260

We need to override them so that they print everything in human readable form for more awkard types such as []byte

[aupiff]

The issues seem to be with pubkey readability. Perhaps the easiest solution if just to make the pubkeys have a type alias over []byte & for that type alias to have a nice String() function