optimize stats using "materialized view"

sqlpage/migrations/003_create_daystats.sql

@@ -0,0 +1,12 @@
+DROP table if exists day_stats;
+
+Create table day_stats as
+SELECT 
+    count(instance_id) as i
+  , sum(users) as u
+  , sum(local_events) as e
+  , sum(local_groups) as g
+  , insertedAt  as x
+  , insertedAt / (3600*24) as j
+FROM stats
+GROUP BY j