Add & use FinalizedCheckpointEpoch() to state

[prestonvanloon]

There were many instances where we were doing state.FInalizedCheckpoint().Epoch which makes a full copy of the checkpoint unnecessarily. Looking at this benchmark, it seems to improve epoch processing by about 30%.

Before

BenchmarkProcessEpoch_2FullEpochs-8           39          34897535 ns/op        23557595 B/op     389336 allocs/op

After

BenchmarkProcessEpoch_2FullEpochs-8           54          25043262 ns/op        18837890 B/op     291021 allocs/op

[codecov]

Codecov Report

Merging #4766 into master will increase coverage by 0.85%.
The diff coverage is 66.66%.

@@            Coverage Diff            @@
##           master   #4766      +/-   ##
=========================================
+ Coverage   55.64%   56.5%   +0.85%     
=========================================
  Files         249     248       -1     
  Lines       18725   18513     -212     
=========================================
+ Hits        10420   10460      +40     
+ Misses       6758    6499     -259     
- Partials     1547    1554       +7