Update BABE specification (Relative Time, Consensus Message Digest, etc.)

[FlorianFranzen]

Updated based on latest research write up.

To be done

• Update algorithm to take probabilistic finalization into account and define Slot-Time(s) again
• Simplify definition of sync epoch (e.g. as multiple of epoch)
• Review and publish Babe randomness algorithm to runtime spec.
• Mention that arrival time of only new blocks are valid (in contrast to block received during sync/catch up).
• Explanation to how to use the algorithm output to estimate future slots.

I assume block arrival times should not be saved to disk, but sync-epochs are rather long (~5h currently) and restarts happen. So for now that would mean it could take validators 5-10h to join the network. This might get even longer if we define sync epochs as a multiple of epochs.

It might also be worth extending the definition of finalized blocks to Grandpa finalized blocks and use the probabilistic finalized blocks only as a fall-back.

This is part of our efforts in #46 and also tracked internally by research here.

Rebased on top of #249 to utilized the improved diffs support.

[FlorianFranzen]

[drskalman]

@FlorianFranzen talks to @hndnklnc to clear out the points mentioned so it can be turned into spec.

[FlorianFranzen]

@hndnklnc wrote up some changes to the protocol, which are tracked in w3f/research-internal#375 and w3f/research-internal#376. I will update the spec accordingly in the coming days.

[burdges]

Added some info for devs in paritytech/substrate#5978 and asked for cumulus to pass clock to parachains in paritytech/cumulus#72 (comment)

[burdges]

Is relative time working yet?

[hndnklnc]

Is relative time working yet?

not yet

[FlorianFranzen]

@burdges @hndnklnc I am getting an issue together with outstanding points to discuss to get an implementation going. Will add you to it once it is ready.

[burdges]

I've added more detail for relative time and approval assignments in paritytech/substrate#7398 but maybe we should extract all this onto one design document, either expanding https://hackmd.io/FY4MCfZCSZGkV-Ulo3VSRg or something new, maybe even being more formal.

All requested changes have been implemented. Thank you for the feedback.