Implements a % cap on staking rewards from era inflation (#1660)

This PR implements an (optional) cap of the era inflation that is
allocated to staking rewards. The remaining is minted directly into the
[`RewardRemainder`](https://github.com/paritytech/polkadot-sdk/blob/fb0fd3e62445eb2dee2b2456a0c8574d1ecdcc73/substrate/frame/staking/src/pallet/mod.rs#L160)
account, which is the treasury pot account in Polkadot and Kusama.

The staking pallet now has a percent storage item, `MaxStakersRewards`,
which defines the max percentage of the era inflation that should be
allocated to staking rewards. The remaining era inflation (i.e.
`remaining = max_era_payout - staking_payout.min(staking_payout *
MaxStakersRewards))` is minted directly into the treasury.

The `MaxStakersRewards` can be set by a privileged origin through the
`set_staking_configs` extrinsic.

**To finish**
- [x] run benchmarks for westend-runtime

Replaces https://github.com/paritytech/polkadot-sdk/pull/1483
Closes https://github.com/paritytech/polkadot-sdk/issues/403

---------

Co-authored-by: command-bot <>

substrate/frame/staking/src/lib.rs

@@ -202,6 +202,12 @@
 //! ```nocompile
 //! remaining_payout = max_yearly_inflation * total_tokens / era_per_year - staker_payout
 //! ```
+//!
+//! Note, however, that it is possible to set a cap on the total `staker_payout` for the era through
+//! the `MaxStakersRewards` storage type. The `era_payout` implementor must ensure that the
+//! `max_payout = remaining_payout + (staker_payout * max_stakers_rewards)`. The excess payout that
+//! is not allocated for stakers is the era remaining reward.
+//!
 //! The remaining reward is send to the configurable end-point [`Config::RewardRemainder`].
 //!
 //! ### Reward Calculation
@@ -897,8 +903,10 @@ impl<Balance: Default> EraPayout<Balance> for () {
 /// Adaptor to turn a `PiecewiseLinear` curve definition into an `EraPayout` impl, used for
 /// backwards compatibility.
 pub struct ConvertCurve<T>(sp_std::marker::PhantomData<T>);
-impl<Balance: AtLeast32BitUnsigned + Clone, T: Get<&'static PiecewiseLinear<'static>>>
-	EraPayout<Balance> for ConvertCurve<T>
+impl<Balance, T> EraPayout<Balance> for ConvertCurve<T>
+where
+	Balance: AtLeast32BitUnsigned + Clone + Copy,
+	T: Get<&'static PiecewiseLinear<'static>>,
 {
 	fn era_payout(
 		total_staked: Balance,
@@ -912,7 +920,7 @@ impl<Balance: AtLeast32BitUnsigned + Clone, T: Get<&'static PiecewiseLinear<'sta
 			// Duration of era; more than u64::MAX is rewarded as u64::MAX.
 			era_duration_millis,
 		);
-		let rest = max_payout.saturating_sub(validator_payout.clone());
+		let rest = max_payout.saturating_sub(validator_payout);
 		(validator_payout, rest)
 	}
 }