Simd 118: add recalculation method and use when booting from snapshot

[CriesofCarrots]

Problem

As per SIMD-0118, partitioned rewards should be recalculated on boot from snapshot. Partition and initial calculation info is available in the EpochRewards sysvar, and as of #1138, Bank::get_epoch_reward_calculate_param_info provides vote-account credits_observed from the EpochStakes cache. This is everything needed to recalculate undistributed stake-reward partitions.

Summary of Changes

Add Bank::recalculate_partitioned_rewards method. Recalculated partitions could differ from the original calculation in two ways:

1. Block partitions that have already been rewarded will be empty. This is because stake-account state is pulled from the StakesCache, which is modified as distribution occurs. (Specifically, stake credits_observed are updated to the new amount, so subsequent calculations determine no distribution needed.) Since only upcoming partitions need action (distribution), this is fine.
2. Rewards within a particular partition may appear in a different order. The runtime makes no guarantees about reward order, either in actual account updates or in block metadata storage, so this is fine.

Use recalculate_partitioned_rewards in Bank::new_from_fields to populate Bank::epoch_reward_status

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 98.81890% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 82.1%. Comparing base (4ae2ca1) to head (f138560).
Report is 12 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #1159    +/-   ##
========================================
  Coverage    82.1%    82.1%            
========================================
  Files         886      886            
  Lines      236439   236581   +142     
========================================
+ Hits       194252   194379   +127     
- Misses      42187    42202    +15     

[jstarry]

Seems useful to report these metrics, are you planning to add this later?

[CriesofCarrots]

Yep. I want to take a look at the metrics separately with the whole implementation in mind. The reporting cadence will need to change, and I don't think all the fields will still be relevant.

[CriesofCarrots]

Rebased to pick up tip of master; changes all in the new, last 4 commits

[joncinque]

Looks great! Just a couple of tiny things, can be addressed separately if more convenient

[CriesofCarrots]

Looks great! Just a couple of tiny things, can be addressed separately if more convenient

Thanks! Went ahead and pushed those here as the last commit. Rebasing to ensure nothing weird upstream.