Dev downtime by epoch

[ghost]

Changes:

• add new params EpochDowntimeIndex and EpochActiveValidationScoreIndex for collect downtime/score data by epoch (default is false);
• add new param epoch for methods GetDowntime and GetValidationScore;
• output downtime and scores for validator+epoch in param from methods GetDowntime and GetValidationScore;

[codecov-io]

Codecov Report

Merging #441 into develop will decrease coverage by 0.24%.
The diff coverage is 2.94%.

@@             Coverage Diff             @@
##           develop     #441      +/-   ##
===========================================
- Coverage    43.48%   43.24%   -0.25%     
===========================================
  Files          163      163              
  Lines        11546    11608      +62     
===========================================
- Hits          5021     5020       -1     
- Misses        6121     6183      +62     
- Partials       404      405       +1

Impacted Files	Coverage Δ
app/store.go	33.33% <ø> (ø)	⬆️
gossip/ethapi_backend.go	0% <0%> (ø)	⬆️
gossip/scores.go	0% <0%> (ø)	⬆️
app/store_scores.go	0% <0%> (ø)	⬆️
gossip/config.go	100% <100%> (ø)	⬆️
evmcore/tx_pool.go	75.85% <0%> (-0.82%)	⬇️
gossip/fetcher/fetcher.go	63.54% <0%> (+1.47%)	⬆️

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