Separate network message for proofs

[dergoegge]

In one of the meetings we shortly discussed whether there should be an extra message just for the proofs or if a new "ublock" message should be used that holds the block and the proof. We decided on the latter.

I think there is case to be made for separate proof messages because blocks and txs could then still be downloaded from regular full nodes which has several benefits:

• less bandwidth stress on bridge nodes when serving compact nodes during IBD
• eclipse attacks are harder to pull off
• (a new node type that only keeps the proofs on disk but prunes the blocks)

Obviously a compact node could always download from regular nodes but without separate proof messages that would essentially mean blocks and txs are downloaded twice (once with proof and once without).

I don't remember why we chose not to go with separate messages, maybe because it would be harder to implement?

I think this is worth another discussion.