[odb-users] persisting objects increases long-term memory usage (with sqlite)

[jose miguel mut]

Hi,

Indeed it was a sqlite issue, one of the suggestions in that
stackoverflow question, was using the `PRAGMA shrink_memory`, which
solved the problem.

I achieved executing the pragma with this code between transactions:
```
        {
            odb::core::connection_ptr c{db->connection()};
            c->execute("PRAGMA shrink_memory");
        }
```

For future users, some more details about the memory numbers in our
project: https://github.com/EBIvariation/vcf-validator/pull/55

Regards