Simd 118: use vote state from EpochStakes in calculation

[CriesofCarrots]

Problem

Stake-reward calculation methods assume that the caller is interested in existing VoteState (specifically, the last epoch in the VoteState::epoch_credits list). This is only true when rewards are calculated at the epoch-boundary block. To support recalculation in later blocks, calculation needs to have access to the VoteState such as it was at the epoch boundary.

The VoteState itself contains previous epoch-credit information, and #1102 was a way to expose this data to calculation methods. However, that approach requires that vote accounts never be destroyed during the period in which recalculation might occur, which would probably necessitate a change to the Vote Program.

Meanwhile, EpochStakes cached in the bank and snapshots already preserves vote account state from the previous epoch.

Summary of Changes

Use the vote accounts from EpochStakes in partitioned epoch rewards calculation.
Note: this works for epoch-boundary calculation because EpochStakes is updated before calculation:

agave/runtime/src/bank.rs

Lines 1423 to 1435 in be80b92

	// Save a snapshot of stakes for use in consensus and stake weighted networking
	let leader_schedule_epoch = self.epoch_schedule.get_leader_schedule_epoch(slot);
	let (_, update_epoch_stakes_time) = measure!(
	self.update_epoch_stakes(leader_schedule_epoch),
	"update_epoch_stakes",
	);
	let mut rewards_metrics = RewardsMetrics::default();
	// After saving a snapshot of stakes, apply stake rewards and commission
	let (_, update_rewards_with_thread_pool_time) = measure!(
	{
	if self.is_partitioned_rewards_code_enabled() {
	self.begin_partitioned_rewards(

Currently includes a check that the operative stakes (from StakesCache) are identical to those from the EpochStakes. This isn't important for the existing implementation, where calculation is still all in the epoch-boundary block and EpochStakes and StakesCache are therefore equivalent, but will be helpful for recalculation. If you'd like me to punt this bit to the next PR, which adds recalculation, let me know. Removed

Closes #1102

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.1%. Comparing base (79ae84e) to head (42d5463).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master    #1138     +/-   ##
=========================================
- Coverage    82.1%    82.1%   -0.1%     
=========================================
  Files         880      880             
  Lines      235632   235653     +21     
=========================================
+ Hits       193649   193654      +5     
- Misses      41983    41999     +16     

[CriesofCarrots]

@joncinque , thanks for all your time on #1102 and sorry for this whiplash 😕 Justin and I talked some on that PR and a little bit in DM and think this is a better direction than using the StakesCache vote accounts for recalculation. He is also investigating updating EpochStakes to store the full stake_state::Stake, which would enable using that as the source of stake accounts for calculation as well.
Happy to sync up with you if it would be helpful to have more detail or context!

[joncinque]

Sorry for the lateness on this, but looks good to me