Fix pending checkpoints submission

[Stefan-Ethernal]

Description

This PR fixes epoch ending blocks detection for submitting pending checkpoints. Prior to this change, for first pending checkpoint block, we were checkpointing epoch-beginning instead of epoch-ending block.

Changes include

• Bugfix (non-breaking change that solves an issue)
• Hotfix (change that solves an urgent issue, and requires immediate attention)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)

Checklist

• I have assigned this PR to myself
• I have added at least 1 reviewer
• I have added the relevant labels
• I have updated the official documentation
• I have added sufficient documentation in code

Testing

• I have tested this code with the official test suite
• I have tested this code manually

[ferranbt]

Can we include any unit test or e2e test to prevent this?

[Stefan-Ethernal]

Can we include any unit test or e2e test to prevent this?

Sure I meant to implement a unit test, although haven't yet made it. Stay tuned 🙂

[ferranbt]

Sure I meant to implement a unit test, although haven't yet made it. Stay tuned

Sorry I thought it was ready for review!

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (feature/v3-parity@16463dd). Click here to learn what that means.
The diff coverage is n/a.

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