Fix #181: Queue adhoc tasks by duration

[djarran]

Description: This resolves issue #181 by changing the SQL used to retrieve the re-engagement modules. No functional changes.

Testing instructions:

0. Cherry pick the changes in Fix #180: Check if adhoc tasks are queued before log output #186 (both can be tested at the same time)
1. Create new test course
2. Enable activity completion
3. Add re-engagement module
4. Under 'Completion conditions', select a reengagement duration of 1 week
5. Add another re-engagement module and select a duration of 1 hour
6. Execute the cron task with php admin/cli/scheduled_task.php --execute=mod_reengagement\\task\\cron_task
7. Confirm that the output message looks like this, where the first module queued should have a higher ID:

Execute scheduled task: Reengagement cron task (mod_reengagement\task\cron_task)
... started 00:18:38. Current memory use 18.1 MB.
Queued re-engagement module: (rid: 2, cmid: 66)
Queued re-engagement module: (rid: 1, cmid: 65)

8. Run the task again and confirm that the tasks were not added to the queue (skipped)

Execute scheduled task: Reengagement cron task (mod_reengagement\task\cron_task)
... started 00:43:37. Current memory use 18.1 MB.
Skipped queueing re-engagement module: (rid: 2, cmid: 66)
Skipped queueing re-engagement module: (rid: 1, cmid: 65)

[gbarat87]

This looks good to me, working as expected