dashboard: more performance tuning

various caching.  make benchmark data a list
in a single per-builder,benchmark record instead
of having one record for each data point.
significant reduction in datastore cpu charges.

R=agl1, agl
CC=golang-dev
https://golang.org/cl/217111

misc/dashboard/godashboard/index.yaml

@@ -13,6 +13,11 @@ indexes:
   - name: __key__
     direction: desc
 
+- kind: BenchmarkResults
+  properties:
+  - name: builder
+  - name: benchmark
+
 - kind: Commit
   properties:
   - name: __key__