Fix active validator indices for next shuffling epoch

[twoeths]

Motivation

• Sync gets stuck on all networks
• Root cause is we precompute active validator indices of next epoch, we should compute for nextEpoch + 1
• @dapplion For spec tests, we always calculate active validator indices when we create EpochContext while this happens in EpochProcess, that's why we didn't see it in spec tests

Description

• Precompute active validator indices for nextEpoch + 1 since active indices for nextEpochShuffling is always available

Closes #2949

Test Result

• Works fine with altair-devnet-3
• Sync mainnet to slot ~1_000_000

[codeclimate]

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[github-actions]

Performance Report

✔️ no performance regression detected

Full benchmark results

Benchmark suite	Current: 38e5cc6	Previous: 7f4f950	Ratio
getCommitteeAssignments - req 1000 vs - 250000 vc	8.8910 ms/op	7.9528 ms/op	1.12
processBlock altair 250000 vs - 7PWei - maxed ops	571.99 ms/op	393.77 ms/op	1.45
processBlock altair 250000 vs - 7PWei - average	216.24 ms/op	138.70 ms/op	1.56
epoch altair - 250000 vs - 7PWei - processInactivityUpdates	2.6245 s/op	2.1928 s/op	1.20
epoch altair - 250000 vs - 7PWei - processRewardsAndPenalties	1.1865 s/op	923.61 ms/op	1.28
epoch altair - 250000 vs - 7PWei - processParticipationFlagUpdates	498.33 ms/op	421.15 ms/op	1.18
processBlock phase0 250000 vs - 7PWei - maxed ops	245.41 ms/op	164.27 ms/op	1.49
processBlock phase0 250000 vs - 7PWei - average	33.075 ms/op	26.546 ms/op	1.25
epoch phase0 - 250000 vs - 7PWei - beforeProcessEpoch	1.8953 s/op	1.5224 s/op	1.24
epoch phase0 - 250000 vs - 7PWei - processRewardsAndPenalties	967.11 ms/op	595.18 ms/op	1.62
epoch phase0 - 250000 vs - 7PWei - processEffectiveBalanceUpdates	157.49 ms/op	92.626 ms/op	1.70
getAttestationDeltas - 250000 vs - 7PWei	126.54 ms/op	113.76 ms/op	1.11
processSlots - 250000 vs - 7PWei - 32 empty slots	3.5395 s/op	3.0137 s/op	1.17
shuffle list - 16384 els	17.554 ms/op	26.145 ms/op	0.67
shuffle list - 250000 els	247.63 ms/op	252.98 ms/op	0.98
getPubkeys - persistent - req 1000 vs - 250000 vc	19.987 us/op	17.315 us/op	1.15
BLS verify - blst-native	2.2993 ms/op	1.8592 ms/op	1.24
BLS verifyMultipleSignatures 3 - blst-native	4.6006 ms/op	3.8092 ms/op	1.21
BLS verifyMultipleSignatures 8 - blst-native	10.185 ms/op	8.2066 ms/op	1.24
BLS verifyMultipleSignatures 32 - blst-native	40.196 ms/op	32.535 ms/op	1.24
BLS aggregatePubkeys 32 - blst-native	50.213 us/op	40.318 us/op	1.25
BLS aggregatePubkeys 128 - blst-native	193.82 us/op	155.65 us/op	1.25
getAttestationsForBlock	223.59 ms/op	173.69 ms/op	1.29
validate gossip signedAggregateAndProof - struct	5.7018 ms/op	4.5049 ms/op	1.27
validate gossip signedAggregateAndProof - treeBacked	5.5845 ms/op	4.4803 ms/op	1.25
validate gossip attestation - struct	5.3435 ms/op	4.2890 ms/op	1.25
validate gossip attestation - treeBacked	2.7614 ms/op	2.2194 ms/op	1.24

by benchmarkbot/action

[dapplion]

Awesome find! Thanks for taking a look