Revert "Downsample duration sum (to calculate total consuming time)"

Having put some more thought in this it doesn't make sense to sum the
total durations as this can be influenced heavily by lots of fast
requests.

This reverts commit 8e2c18f9.

queries/grape/grape_transaction_timings_per_action.txt

 CREATE CONTINUOUS QUERY grape_transaction_timings_per_action ON $DATABASE
 BEGIN
-    SELECT sum("duration") AS duration_sum, 
-        mean("duration") AS duration_mean,
+    SELECT mean("duration") AS duration_mean,
         percentile("duration", 95) AS duration_95th,
         percentile("duration", 99) AS duration_99th,
         mean(sql_duration) AS sql_duration_mean,

queries/rails/rails_transaction_timings_per_action.txt

 CREATE CONTINUOUS QUERY rails_transaction_timings_per_action ON $DATABASE
 BEGIN
-    SELECT sum("duration") AS duration_sum,
-        mean("duration") AS duration_mean,
+    SELECT mean("duration") AS duration_mean,
         percentile("duration", 95) AS duration_95th,
         percentile("duration", 99) AS duration_99th,
         mean(sql_duration) AS sql_duration_mean,