[GPoS] Clear stakers should not remove online validators

[badkk]

Problem

If chain in a danger situation(corner case), it won't update any staking related data structures(minimum validator protection)

crust/cstrml/staking/src/lib.rs

Lines 1512 to 1528 in d1977ad

	<tee::Module<T>>::update_identities();
	Self::clear_stakers();
	let validators: Vec<(T::AccountId, Validations<T::AccountId>)> =
	<Validators<T>>::iter().collect();
	let validator_count = validators.len();
	let minimum_validator_count = Self::minimum_validator_count().max(1) as usize;
	if validator_count < minimum_validator_count {
	// There were not enough validators for even our minimal level of functionality.
	// This is bad.
	// We should probably disable all functionality except for block production
	// and let the chain keep producing blocks until we can decide on a sufficiently
	// substantial set.
	// TODO: #2494(paritytech/substrate)
	return None;
	}

However, we should not clear online validator's stakers, cause rewards based on staker(snapshot) status:

crust/cstrml/staking/src/lib.rs

Lines 1426 to 1433 in d1977ad

	// 2. Staking payout
	let current_era = Self::current_era().unwrap_or(0);
	let total_staking_payout = Self::staking_rewards_in_era(current_era);
	let total_stakes = Self::total_stakes();
	<Stakers<T>>::iter().for_each(|(v, e)| {
	let staking_reward = Perbill::from_rational_approximation(to_num(e.total), to_num(total_stakes)) * total_staking_payout;
	total_imbalance.subsume(Self::reward_validator(&v, staking_reward));
	});

Proposal

We can zip Stakers with Validators, remove those offline validator's staker