Align the changes to rewards made for "private chain" and for "2.0"

[RitaMAllenCA]

Recently we did a merge of dev to feat-2.0. Because both branches tackled with rewards calculation, there is a TODO added in era_supervisor.rs to remind that we need to verify if we support the compute_rewards chainspec flag correctly in in 2.0. Details here.

TODO:

1. Merge feat-2.0 into feat-signature-rewards (bring data extensibility and private chain changes into rewards feature branch)
2. Check if the private chain changes to rewards work correctly with the rewards changes on the feature branch
3. Optionally (but preferably) merge feat-signature-rewards back to feat-2.0