This allows running queries outside of transactions, which is useful to
avoid the overhead of transaction management (in terms of RTT and
isolation levels).

We call `_update_stream_positions_table_txn` a lot, which is an UPSERT
that can conflict in `REPEATABLE READ` isolation level. Instead of doing
a transaction consisting of a single query we may as well run it outside
of a transaction.

Co-authored-by: Richard van der Hoff <[email protected]>