genesis-test: Create DB snapshot using full tx isolation

[mitjat]

As discussed on Slack, mainnet-indexer-statecheck (the cron job that runs genesis_test on consensus) consistently fails. A possible cause for failure is that the test creates an inconsistent snapshot of the indexer state, due to not using a strong enough DB tx isolation level.

This PR uses SERIALIZABLE, the strongest tx isolation level, for snapshot creation.

More about DB tx isolation levels:
https://www.postgresql.org/docs/current/transaction-iso.html

I created a dev docker image and I'm deploying this now to see what the effect is.

kubectl -n monitoring create job --from=cronjobs/mainnet-indexer-statecheck manual-indexer-statecheck-mitjat

[mitjat]

A test run completed. Unfortunately, it still fails in the same way as before: some nonces, allowances etc are off.
We should still merge this PR; it's a legitimate bug in the test harness.