Change -all_time to only affect logs we haven't seen before

It's more useful this way - there's no sense in scanning logs we've
already scanned.

I need a better name for this switch, though.

cmd/common.go

@@ -167,10 +167,10 @@ func Main (argStateDir string, processCallback ctwatch.ProcessCallback) {
 		}
 
 		var startIndex uint64
-		if *allTime {
-			startIndex = 0
-		} else if prevSTH != nil {
+		if prevSTH != nil {
 			startIndex = prevSTH.TreeSize
+		} else if *allTime {
+			startIndex = 0
 		} else {
 			startIndex = latestSTH.TreeSize
 		}