DigestItem refactoring for swappable consensus

[rphmeier]

Related to #1365

Right now, the DigestItem::Seal in sr-primitives is very Aura-specific. As part of Swappable Consensus · Issue #1304 · paritytech/substrate · GitHub we are moving a lot of stuff to be based on ConsensusEngineIds. For BABE this would be the array ['b'B', b'A', b'B', b'E'].

Currently DigestItem::<SealSignature>::Seal(u64, SealSignature)

We'd like it to be defined as DigestItem::Seal(ConsensusEngineId, Vec<u8>). This will lead to the Signature parameter being removed from DigestItem and all containing structs (this is a win!).

Unfortunately, that's not backwards compatible with the old Aura implementation and will break things.

To make it backwards compatible, we will originally add this as DigestItem::Consensus(ConsensusEngineId, Vec<u8>) and write a fn start_aura2.

[Demi-Marie]

@rphmeier does this still depend on #1365?

[rphmeier]

No, I don't think so. Removed that from the issue description.