step 1 double the freq from once a week to once every 3days (#11769)

Increase the statistics report frequency from once a week to once in 3
days.
The idea is to double it in slow steps to make sure the Heroku can
withstand it.

Goal is to get to once every few hours.

server/fleet/statistics.go

@@ -54,5 +54,5 @@ type HostsCountByOSVersion struct {
 }
 
 const (
-	StatisticsFrequency = time.Hour * 24 * 7
+	StatisticsFrequency = time.Hour * 24 * 3
 )