[Staking] Rate limit how often validators can change commission

[Ank4n]

Context

Related forum discussion:
https://forum.polkadot.network/t/validators-flipping-their-commision-twice-in-an-era-and-cheating-nominators/16569/14

We’ve observed validators changing their commission rate shortly before and after an era boundary. This can result in a low effective commission for the era (e.g. 0%), while UIs display a higher current commission (e.g. 10%), which is confusing for nominators.

Mitigation

Off-chain: UIs can display the actual historical per-era commission (we store the last 84 eras on-chain) instead of only the current, mutable commission.

Optional on-chain:
Consider rate-limiting commission changes (e.g. allowing updates at most once per era) to reduce commission flipping.

At the moment, this feels low priority, and we could likely avoid on-chain changes unless there is strong community demand.

[burdges]

We could make them update the commission well in advance I guess, but maybe a rate limit is simpler.