Address last use of get_last_rowid

The `get_last_rowid` function is being called [here](https://github.com/AnswerDotAI/sqlite-minutils/blob/main/sqlite_minutils/db.py#L276-L279) in `insert_chunk`: https://github.com/AnswerDotAI/sqlite-minutils/blob/main/sqlite_minutils/db.py#L2992

That means that inserts are still vulnerable to corruption due to concurrency, transactions, and vacuums.

Read sqlite_minutils/db.py around lines 276-279 and 2992, tracing get_last_rowid and insert_chunk. Determine how the last use can be addressed without leaving inserts vulnerable during concurrency, transactions, or vacuums; done means insert_chunk no longer relies on get_last_rowid in those cases.

Written by the indexing model from the issue text.