Implement infrastructure for long upgrade processes

When performing significant upgrade tasks (like CRM-9923 or reorganizing activity-contact relationship tables) on large datasets, some common PHP execution environments will abend. To better manage this problem, we should

1. Provide a queue abstraction
2. Provide a queue implementation which executes tasks interactively (similar to Drupal's Batch API)
3. Modify the upgrade logic to submit bite-sized upgrade tasks to the queue and to use the queue
4. Modify the upgrade logic to handle the special case of bootstrapping the queue table(s)