Clamp user_exercise_summary.time_taken to 600 seconds

Summary:
(Experimenting with adding everyone to this review)

This is to maintain parity with user_video_summary.q, which clamps on 1800 seconds.  I chose 600 seconds, because that's what we do in in webapp:

```
/Users/mattfaus/dev/webapp/consts.py:
   52  # Maximum time we're willing to report that a user worked on a single
   53  # problem, in seconds
   54: MAX_WORKING_ON_PROBLEM_SECONDS = 600

/Users/mattfaus/dev/webapp/content/exercise_models.py:
 2976          # oddities.
 2977          return max(0,
 2978:                 min(consts.MAX_WORKING_ON_PROBLEM_SECONDS, self.time_taken))
```

Test Plan:
Run this query manually (note future partition, old data so it will be overwritten):

```
INSERT OVERWRITE TABLE user_exercise_summary PARTITION (dt='2013-10-23')
SELECT
  parsed.user,
  parsed.exercise,
  SUM(IF(parsed.time_taken < 0, -- Clamp in [0, 600) for sanity
    0,
    IF(parsed.time_taken > 600,
      600,
      parsed.time_taken
    ))),
  SUM(parsed.correct),
  SUM(parsed.wrong),
  MAX(IF(parsed.proficient, 1, 0)) = 1
FROM (
  SELECT
    get_json_object(ProblemLog.json, '$.user') AS user,
    get_json_object(ProblemLog.json, '$.exercise') AS exercise,
    get_json_object(ProblemLog.json, '$.time_taken') AS time_taken,
    IF(get_json_object(ProblemLog.json, '$.correct') = "true", 1, 0) AS correct,
    IF(get_json_object(ProblemLog.json, '$.correct') != "true", 1, 0) AS wrong,
    get_json_object(ProblemLog.json, '$.earned_proficiency') = "true"
      AS proficient
  FROM ProblemLog
  WHERE ProblemLog.dt = '2013-10-15'
) parsed
GROUP BY parsed.user, parsed.exercise;
```

Then, queried, to make sure there were no rows with >600 seconds per problem.

```
select (time_spent / (num_correct+num_wrong))
from user_exercise_summary
where dt = '2013-10-23'
and (time_spent / (num_correct+num_wrong)) > 590
```

There were 5067 rows that matched this query, most of which were capped at 600, as expected.

Reviewers: jace, jascha, eliana, tom, mattfisher

Reviewed By: jace

Differential Revision: http://phabricator.khanacademy.org/D4761