Support filtering in /user/tasks on both task result and tags

[GoogleCodeExporter]

Use case:
Be able to filter all tasks with: task result is failed and task has tag "gpu:1"

This requires improving the composite indexes, which requires getting rid of 
the old created_ts index. created_ts is now unnecessary with the new native key 
sorting. In particular, indexes like this one:
- kind: TaskResultSummary
  properties:
  - name: failure
  - name: state
  - name: created_ts

can be stripped to:
- kind: TaskResultSummary
  properties:
  - name: failure
  - name: state

and this index must be removed:
- kind: TaskResultSummary
  properties:
  - name: state
  - name: created_ts

Then we can add instead at lower cost:
- kind: TaskResultSummary
  properties:
  - name: failure
  - name: state
  - name: tags

- kind: TaskResultSummary
  properties:
  - name: state
  - name: tags

Original issue reported on code.google.com by maruel@chromium.org on 27 Jan 2015 at 3:17