Expose the peer that delivered the message

[aschmahmann]

It may be useful for peers that want to layer protocols on top of pubsub to know which peer actually delivered a message to them.

We already have this information available in the from field of the RPC:

go-libp2p-pubsub/pubsub.go

Lines 144 to 149 in 2247a54

	type RPC struct {
	pb.RPC
	// unexported on purpose, not sending this over the wire
	from peer.ID
	}

so this would mean making the information publicly available in the Message struct:

go-libp2p-pubsub/pubsub.go

Lines 136 to 138 in 2247a54

	type Message struct {
	*pb.Message
	}

[aschmahmann]

@raulk @vyzo @ZenGround0

Do we think this is sufficiently reusable such that we're better off recommending this approach rather than the hackier, but already available, approach of using the validators? Note, that the validators take the peer that delivered the message as an input so we actually already have an avenue for getting the from into application space.

[vyzo]

Using the validators is just too hacky, let's expose from.

[raulk]

Closed in #218.