Reduce CPU usage

We take advantage of the fact that dnscache uses a single space
character to separte fields, allowing us to use the simpler `strings.Split`
function instead of `strings.Fields`. When tested on real data using the
`Timestamped` benchmark, there was a 2x speedup.

main.go

@@ -126,7 +126,7 @@ func (l *logReader) run() {
 	}
 	scanner := bufio.NewScanner(l.src)
 	for scanner.Scan() {
-		line := strings.Fields(scanner.Text())
+		line := strings.Split(scanner.Text(), " ")
 		if len(line) < 1 {
 			continue
 		}