Able to chill inactive validator

[aurexav]

Add a new extrinsic chill_inactive_validator and a new storage config ChillInactiveValidatorThreshold:

• Anyone can call this function.
• The validator's stash account should be provided.
• The proof should be a list of consecutive sorted era indexes where the validator has
  not produced any blocks.
• This will only be successful if the validator has not produced any blocks in ChillInactiveValidatorThreshold consecutive eras.
• If the ChillInactiveValidatorThreshold is not set, it will fallback to Config::HistoryDepth. It's almost the same as current on-chain logic. The inactive validator is almost impossible to be chilled by others, unless it missed 84 eras.

Closes #5674.

---

Polkadot address: 156HGo9setPcU2qhFMVWLkcmtCEGySLwNqa3DaEiYSWtte4Y

[bkchr]

Suggested change

	consecutive_inactives = 0;
	break;

Invalid proof.

[aurexav]

Same as above.

[bkchr]

Suggested change

	consecutive_inactives = 0;
	continue;
	break;

Invalid proof.

[aurexav]

This also makes the rule strict. Every point needs to be valid. However, it’s acceptable if we anticipate this.

[bkchr]

Suggested change

	// Check if the ear exists.
	if !ErasRewardPoints::<T>::contains_key(era) {
	consecutive_inactives = 0;
	continue;
	// Check if the era exists.
	if !ErasRewardPoints::<T>::contains_key(era) {
	break;

Invalid proof.

[aurexav]

Validator inactive for [0, 1, 2, 3].
Threshold 3.
Era 0 gets clipped from the history at the moment.

This will reject [0, 1, 2, 3]. Is this expected?