Figure out a better way to deal with sync_id

[compojoom]

We use the sync_id to know what ethindex updates we've processed.

However if we rest ethindex (drop tables, create tables, import abi and resync) it can happen that we forget to also update the last_sync_id file and the relay would try to sync from a non-existing place. This causes the balance updates in the app to be wrong.

We should either figure out a better way to deal with it or find another way to know the last synced state.