Trying to reduce memory allocation. logQueryArgs is called even when

using the default discardLogger. This allocates quite a bit of memory.
Created a default dlogger instance and avoid logging code when that's our
logger. I couldn't think of a better implementation without touching the
Logger interface.

On 100 000 inserts, this brought the allocation down to 58MB from 122MB.
It should also result in some small performance gains.

query.go

@@ -68,6 +68,10 @@ func (rows *Rows) close() {
 
 	rows.closed = true
 
+	if rows.logger == dlogger {
+		return
+	}
+
 	if rows.err == nil {
 		endTime := time.Now()
 		rows.logger.Info("Query", "sql", rows.sql, "args", logQueryArgs(rows.args), "time", endTime.Sub(rows.startTime), "rowCount", rows.rowCount)