Make build_id insertions add histograms correctly

Bug 1403994

Previously, old build_id histograms would merge with new ones
by just adding; but instead we want to first add the histograms,
then the sums and counts separately.

Unfortunately this means that [0] did not fix historical build_id
aggregates. Those are broken in the db for good.

[0] https://github.com/mozilla/python_mozaggregator/pull/57

mozaggregator/sql.py

@@ -53,7 +53,7 @@ begin
 
     -- Update existing tuples and delete matching rows from the staging table
     execute 'with merge as (update ' || tablename || ' as dest
-                            set histogram = aggregate_arrays(dest.histogram, src.histogram)
+                            set histogram = aggregate_histogram_arrays(dest.histogram, src.histogram)
                             from ' || stage_table || ' as src
                             where dest.dimensions = src.dimensions
                             returning dest.*)