[DBTNG] Properly force default database in the dblog module

dblog_watchdog() already forces the use of the default database, but it does so by database switching. That is so last version. These days we can request the database directly.

While I was at it I also converted dblog_watchdog() to use a built Insert query. I did not convert the rest of the module to DBTNG yet, as there's some tricksy Select work going on for the admin page that is pending on #299267: Add "Extender" support to SELECT query builder to make the Select builder even more flexible.