[do not merge] Decouple rewards from ledger

[Ank4n]

Related to paritytech/polkadot-sdk#439.

This PR changes the way we prevent double claim for validator rewards. We keep track of EraRewardPoints for each validator for current era, and in subsequent eras, when validator reward is claimed, their reward points are dropped from EraRewardPoints.

Since we store 84 eras of past reward claim history, in order to avoid doing a large migration (total nominator count * history_depth), we will do a lazy migration. After this change, no new data will be written to StakingLedger.legacy_claimed_rewards (renamed from claimed_rewards). Instead for any era going forward, we will just drop the EraRewardPoints for the validator as discussed above. When distributing rewards, we check StakingLedger.legacy_claimed_rewards to make sure if any past claim has been recorded there.

After 84 eras. we can be fully confident that StakingLedger.legacy_claimed_rewards has no relevant history that we need to preserve and we can do a new migration to remove it from ledger as well as clean up the code.

Tasks

• add test to verify claimed_rewards is checked before paying out stakers.
• stop initialising claimed_rewards for new stakers.
• clean up exisiting tests.
• re-benchmark.
• Tracker issue to clean up claimed_rewards code, Tracker issue for cleaning up old non-paged exposure logic in staking pallet polkadot-sdk#433.

[Ank4n]

/cmd queue -c bench-bot $ pallet dev pallet_staking

[command-bot]

@Ank4n https://gitlab.parity.io/parity/mirrors/substrate/-/jobs/2213163 was started for your command "$PIPELINE_SCRIPTS_DIR/bench-bot.sh" pallet dev pallet_staking. Check out https://gitlab.parity.io/parity/mirrors/substrate/-/pipelines?page=1&scope=all&username=group_605_bot to know what else is being executed currently.

Comment /cmd cancel 151-d187a857-8c12-450e-9b24-f00660f31e81 to cancel this command or /cmd cancel to cancel all commands in this pull request.

[command-bot]

@Ank4n Command "$PIPELINE_SCRIPTS_DIR/bench-bot.sh" pallet dev pallet_staking has finished. Result: https://gitlab.parity.io/parity/mirrors/substrate/-/jobs/2213163 has finished. If any artifacts were generated, you can download them from https://gitlab.parity.io/parity/mirrors/substrate/-/jobs/2213163/artifacts/download.

[kianenigma]

Suggested change

	fn get_era_reward_points(
	fn temp_get_era_reward_points(

[kianenigma]

since this can be removed later, right?

[kianenigma]

Looks mostly good, but I want to give it an in-depth review later. Also want to look into the tests again. Would be good if you revise the tests as well, as the existing ones might be mediocre at best.

[Ank4n]

While this PR will solve the existing issue of moving rewards away from StakingLedger, it will be incompatible with multi-block rewards payout PR that I am working on. I am inclined to do these changes as part of the other PR to make sure it works well with multi-block rewards.

I will wait for comments/opinions but will probably close it in couple of days.