Run import process inside a SQL transaction

[ittner]

Start a SQL transaction before importing the back and commit it at the end of the process. This has no much effect when using in-memory databases, but makes everything really faster when not, as we don't need to persist every individual statement independently.

[pajowu]

This looks great. I wanted to do that initially but couldn't figure it out properly. I will test this in the next days and hope we can merge that. If this works reliably we can get rid of the in memory db option alltogether 🥳