[indexer] use beginning of epoch system state instead of end of epoch

[emmazzz]

Description

As titled, making sure that system state info is not overwritten at the end of the epoch.

Test plan

How did you test the new or updated feature?

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Release notes

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• [ ] Protocol:
• [ ] Nodes (Validators and Full nodes):
• [ ] Indexer:
• [ ] JSON-RPC:
• [ ] GraphQL:
• [ ] CLI:
• [ ] Rust SDK:

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[stefan-mysten]

Does this change affect any of the validator APY logic in GQL?

[stefan-mysten]

@emmazzz Any plans to merge this before the new release?

[github-actions[bot]]

This PR is stale because it has been open 60 days with no activity. Remove stale label or comment or this will be closed in 7 days.