Avoid recomputing state.previous_epoch

[paulhauner]

Issue Addressed

NA

Proposed Changes

Much to my surprise, BeaconState::previous_epoch has been showing up in flamegraphs! This PR removes repeated calls to it.

Benchmarks

Benchmarks on a recent mainnet state using #3252 to get timing.

These tests were run with optimisations #3254 and #3255 already applied.

Without this PR

lcli skip-slots --state-path /tmp/state-0x3cdc.ssz --partial-state-advance --slots 32 --state-root 0x3cdc33cd02713d8d6cc33a6dbe2d3a5bf9af1d357de0d175a403496486ff845e --runs 10
[2022-06-09T08:56:29Z INFO  lcli::skip_slots] Run 0: 104.628252ms
[2022-06-09T08:56:29Z INFO  lcli::skip_slots] Run 1: 83.104139ms
[2022-06-09T08:56:30Z INFO  lcli::skip_slots] Run 2: 84.025086ms
[2022-06-09T08:56:30Z INFO  lcli::skip_slots] Run 3: 84.747254ms
[2022-06-09T08:56:30Z INFO  lcli::skip_slots] Run 4: 83.197275ms
[2022-06-09T08:56:30Z INFO  lcli::skip_slots] Run 5: 83.390297ms
[2022-06-09T08:56:30Z INFO  lcli::skip_slots] Run 6: 83.057175ms
[2022-06-09T08:56:30Z INFO  lcli::skip_slots] Run 7: 85.289398ms
[2022-06-09T08:56:30Z INFO  lcli::skip_slots] Run 8: 83.081438ms
[2022-06-09T08:56:30Z INFO  lcli::skip_slots] Run 9: 82.661783ms

With this PR

lcli skip-slots --state-path /tmp/state-0x3cdc.ssz --partial-state-advance --slots 32 --state-root 0x3cdc33cd02713d8d6cc33a6dbe2d3a5bf9af1d357de0d175a403496486ff845e --runs 10
[2022-06-09T08:57:59Z INFO  lcli::skip_slots] Run 0: 73.898678ms
[2022-06-09T08:57:59Z INFO  lcli::skip_slots] Run 1: 75.536978ms
[2022-06-09T08:57:59Z INFO  lcli::skip_slots] Run 2: 75.176104ms
[2022-06-09T08:57:59Z INFO  lcli::skip_slots] Run 3: 76.460828ms
[2022-06-09T08:57:59Z INFO  lcli::skip_slots] Run 4: 75.904195ms
[2022-06-09T08:58:00Z INFO  lcli::skip_slots] Run 5: 75.53077ms
[2022-06-09T08:58:00Z INFO  lcli::skip_slots] Run 6: 74.745572ms
[2022-06-09T08:58:00Z INFO  lcli::skip_slots] Run 7: 75.823489ms
[2022-06-09T08:58:00Z INFO  lcli::skip_slots] Run 8: 74.892055ms
[2022-06-09T08:58:00Z INFO  lcli::skip_slots] Run 9: 76.333569ms

Additional Info

• Blocked on [Merged by Bors] - Optimise per_epoch_processing low-hanging-fruit #3254 and Use Vec in SingleEpochParticipationCache rather than HashMap #3255

[paulhauner]

I'll reopen another PR with this change combined with others.