[LiskHQ/lisk] Multiple rows for one transaction ID in votes table of snapshots (#2054)

> Based on earlier discussion we concluded that the duplicate rows for transactions sub-tables were generated by some bug in past while there was no atomic block write, and carried forward in all snapshots.
> Since after the atomic block write its not possible to have such scenario, where block processing failed, and re-processing same block caused duplicate rows.

I don’t know what caused the invalid data I described in https://github.com/LiskHQ/lisk/issues/2054#issuecomment-413338673 but it cannot be «some old bug» as the snapshot provider published clean 1.x Testnet snapshots for weeks before the incident happened.

But a `UNIQUE` constraint will probably tell us which code is responsible for such problems.