staking: use validator exchange rate to compute rewards

[erwanor]

Submitted for independent review and testing, should fix #3802.

Testing that this change work is a little bit tricky, here are the steps that I have taken just now:

1. I setup a devnet with short block time (300ms), and short epoch durations (50 blocks).
2. The default validator has 100% of the total active stake and a 100% commission rate.
3. On the v0.66.0 tag, this experimental setups leads to a chain halt on epoch 2 (~ 25 seconds after start).

[cronokirby]

I gave this a thorough synchronous review with @erwanor, and am convinced that this is correct, and fixes other bugs beyond the circuit breaker, like a validator getting too much in commissions if slashed.

[cratelyn]

seconding the approval, as another participant in group review today. ⭐