Substrate: Switch to BABE consensus algorithm

[cernicc]

Switch from Aura consensus algorithm to BABE. We need BABE as a prerequisite for secure randomness on chain.

https://docs.substrate.io/learn/consensus

[cernicc]

Hmm. While researching this. The Babe consensus algorithm was developed before the Aura. We are using Cumulus as an substrate extension, it helps us to easier make a Polkadot compatible parachain.

Cumulus decided not to support Babe. paritytech/cumulus#116 . I am not sure what changes would be needed to use babe without the cumulus part.

The decision if this should be done will depend on the decision on this. Switching to BABE is not practical for the parachain.