Expose the peer that propagates a message to the recipient

[aschmahmann]

This is a solution for #215. We were doing a lot of passing around (src, Message) to avoid exposing the source next to the message, so I hope that solving #215 is a good thing 😬.

I haven't removed all the places where we pass both src and Message, but got most of the internal ones.

[raulk]

This looks like a pretty good rationalisation to me anyway, even if we didn't have to address #218.

[raulk]

Let's wait for @vyzo to sign-off too.

[vyzo]

LGTM

[vyzo]

Test failure is systemic, @aschmahmann you need to go fmt!

*** go fmt ./...

ERROR: some files not gofmt'ed:

./floodsub_test.go

[aschmahmann]

@aschmahmann you need to go fmt!

lol that's what I get for trying to rename a variable using the Github text editor