## Description 

quick followups of https://github.com/MystenLabs/sui/pull/15434

also updated system state at the end of epoch

## Test Plan 

local run and check that last_epoch's system state is always the same as
the current epoch via query
```
SELECT a.epoch, a.system_state, b.epoch, b.system_state
FROM epochs a, epochs b
WHERE a.system_state = b.system_state AND a.epoch < b.epoch
```

---
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### Type of Change (Check all that apply)

- [ ] protocol change
- [ ] user-visible impact
- [ ] breaking change for a client SDKs
- [ ] breaking change for FNs (FN binary must upgrade)
- [ ] breaking change for validators or node operators (must upgrade
binaries)
- [ ] breaking change for on-chain data layout
- [ ] necessitate either a data wipe or data migration

### Release notes