[NPoS] Mint fixed portion of inflation directly into the treasury

[gpestana]

It's been proposed and discussed a change to the current inflation model where a fixed portion of the inflation is minted directly into the treasury, while the remaining portion is distributed as rewards as per the current mechanism. The main goal is to ensure the sustainability of the treasury in the long term (more info in this thread).

In addition, it would be helpful to allow governance to change the percentage of inflation minted into the treasury.

Implementation suggestions

1. fn era_payout calculates the portion of payout that is distributed among stakers (validator_payout) and the treasury based on perthing TreasuryFraction, in storage.
2. The validator payout stored in ErasValidatorReward is total_payout - total_payout * TreasuryFraction. From here on, the logic of the stakers' rewards remains the same.
3. The treasury portion for that era is minted at the end of the era into Treasury::account_id() (another option would be to cache treasury rewards and mint into the treasury pot every X eras, but probably not worth the effort/storage).
4. New extrinsic to update TreasuryFraction perthing, gated by root.

[kianenigma]

It would be good to be more forward looking and design something for that that would also work for a parachain. Would probably mean taking away the minting from staking and doing it elsewhere.

[Polkadot-Forum]

This issue has been mentioned on Polkadot Forum. There might be relevant details there:

https://forum.polkadot.network/t/revising-polkadots-ideal-staking-rate-calculation/3897/1

[Polkadot-Forum]

This issue has been mentioned on Polkadot Forum. There might be relevant details there:

https://forum.polkadot.network/t/adjusting-polkadots-ideal-staking-rate-calculation/3897/6

[gpestana]

For the record, we'll implement these changes using the parameters pallet (under development) (#2061) so that the era payout parameters can be changed using selected origins.