dashboard: lots of caching to avoid datastore queries

reorganize benchmark computation so that it is
incremental.  if it times out, it doesn't lose the
pieces it already computed, so that next time it
has a fighting chance to finish.

R=agl1, agl
CC=golang-dev
https://golang.org/cl/216046

misc/dashboard/godashboard/index.yaml

 indexes:
 
-# AUTOGENERATED
-
-# This index.yaml is automatically updated whenever the dev_appserver
-# detects that a new type of query is run.  If you want to manage the
-# index.yaml file manually, remove the above marker line (the line
-# saying "# AUTOGENERATED").  If you want to manage some indexes
-# manually, move them above the marker line.  The index.yaml file is
-# automatically uploaded to the admin console when you next deploy
-# your application using appcfg.py.
+- kind: BenchmarkResult
+  ancestor: yes
+  properties:
+  - name: builder
+  - name: __key__
+    direction: desc
 
 - kind: BenchmarkResult
   ancestor: yes
@@ -20,3 +17,14 @@ indexes:
   properties:
   - name: __key__
     direction: desc
+
+# AUTOGENERATED
+
+# This index.yaml is automatically updated whenever the dev_appserver
+# detects that a new type of query is run.  If you want to manage the
+# index.yaml file manually, remove the above marker line (the line
+# saying "# AUTOGENERATED").  If you want to manage some indexes
+# manually, move them above the marker line.  The index.yaml file is
+# automatically uploaded to the admin console when you next deploy
+# your application using appcfg.py.
+