[syslog-ng] [PATCH (3.4) 0/3] afsql code cleanups

The patches following this mail are my attempts at cleaning up the afsql driver here and there, mostly afsql_dd_insert_db(), in preparation for moving towards LogThreadedDestDriver (which will be posted separately a bit later). The migration is possible without these patches, but these are worth applying even without the LogThreadedDestDriver patches, as these present no real architectural change, just a little refactoring. I think the code could be cleaned up further, but for the time being, I only did what I thought necessary to prepare for migration to LogThreadedDestDriver.