[DB] Optimisations

[frisitano]

Overview

We should consider the database access patterns and introduce indexes or other optimisations to make the database more efficient. One low hanging fruit would be to introduce an index on the batch hash. Given that our dataset of medium size (~360k batches / ~1m L1 messages) it could benefit from indexes but even without them it should be pretty fast, and as such, this is of low/medium priority.