Fix Off By One Attestation Bug

[terencechain]

This is part of #1542 bug fixes

In this PR we fixes the by one attestation issue as referenced in:
ethereum/consensus-specs#627
ethereum/consensus-specs#618

[codecov]

Codecov Report

Merging #1619 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1619   +/-   ##
======================================
  Coverage    71.1%   71.1%           
======================================
  Files          94      94           
  Lines        6618    6618           
======================================
  Hits         4706    4706           
  Misses       1502    1502           
  Partials      410     410

[rauljordan]

How come this wasn't caught by tests?

[terencechain]

How come this wasn't caught by tests?

How to reproduce this:
1.) attestation.data.slot % EPOCH_LENGTH == 0
2.) let epoch state update to not happen
3.) get_epoch_start_slot returns a new slot from a new epoch (rather than previous epoch)

It's not so much of a unit test regression, it's more so a e2e test regression, our e2e test suite is quite minimal we will need to enhance it