Extend network protocol with justification announcements

[rphmeier]

Just as new blocks are announced and gossiped on the network, new justifications (if produced behind the head) should be as well.

The sync protocol needs to be provided with an object that knows how to check + import justifications (this could go on the BlockImport trait)

Altering the finality notification stream slightly should make issuing such broadcasts easy to do.

[rphmeier]

The key-thing here is making sure that justification announcements propagate, so if I import a valid justification I also reannounce it to my peers.

Probably the hardest thing is not announcing too many justifications and not reannouncing any justifications that are lower than our most recent finalized block.

[tomaka]

Is the issue still relevant? Why do we want to announce justifications? To have them available quicker?

The sync protocol needs to be provided with an object that knows how to check + import justifications (this could go on the BlockImport trait)

This is done.

[andresilva]

It isn't strictly necessary anymore, full clients can just rely on the GRANDPA commits to stay up-to-date to finality. This is what we are doing with the observer already.

It's probably mainly useful for the light client, so that it can track finality in between validator set changes (validator set change blocks are the only ones required to be to be finalized and that the light client will fetch justifications for).

Either way it's not something I see us implementing soon, so if we want to keep the issue let's push to a later milestone. I'll also unassign myself in case someone else wants to take it.