Switch to relay chain's babe randomness

[JoshOrndorff]

Now that https://github.com/paritytech/cumulus/pull/1083 landed, it should be possible to use the relay chain's randomness beacon instead of the parachain's collective flip (which should be removed).

cc @4meta5

[4meta5]

I think we still need https://github.com/paritytech/cumulus/pull/1135 to read the BABE randomness from the relay state proof. We do have the proof in parachain-system storage now.

[librelois]

@4meta5 this is something that is still relevant to do, do we have a JIRA issue for this?

[4meta5]

Yes, it is open -- MOON-1812

From that ticket:

randomness-collective-flip is low entropy. If we replace it, we need something similarly low entropy. Relay chain's epoch randomness is low entropy but only changes once per epoch. Parachain's per-block VRF randomness changes every block but is high entropy.

Combining them somehow may be suitable, this might be a good place to discuss how that ought to be done

[crystalin]

This can finally be closed since we moved to our own randomness #1956 🚀