Relies on:

• add result field and propagate and store errors in tasks #1154
• refactor run events; emit them to sync service sdk-go#31

We should either first merge these PRs, or merge this into tasks-status and do an overall review.

This PR is:

1. Adding a kill command, so that we can cancel tasks that are currently processing or scheduled.
2. Refactoring Task package - making impl. details private (such as prefixes for the LevelDB schema) and unifying the names between the in-memory Queue and the Task.
3. Explicitly adding a StateCanceled for tasks.
4. Extracting task timeout as a config. We now have two timeouts:
   a) task includes both the build and run duration of a task. Consider it as a global timeout.
   b) run is the timeout of the run part of a test plan. It should be less that task timeout.
5. Adding user field to the [client] testground config. This field is attached to tasks so that we identify who triggered a task.