Verify that activeEpochNo field in PoolUpdate table match the actual behavior of the ledger rules

Previously (before `db-sync 10.0`), the value inserted in the `activeEpochNo` did not match the actual behavior of the ledger rules.

If the pool is found in `_pParams` of `PState` before the current block it's `epoch + 3` (retirements in the current block are irrelevant) if it's not found, but there is already another `PoolReg` in the current block it's `epoch + 3` again (we can check this from the db). Any other case is `epoch + 2`.

Related `db-sync` issue: https://github.com/input-output-hk/cardano-db-sync/issues/610