Support Heuristics / Branch Prediction

[state-hiu]

• [x] Celery Instructions / http://celery.readthedocs.org/en/latest/django/first-steps-with-django.html
• [x] settings.py
• [x] tasks
• [ ] Priority queue for background tile requests / http://docs.celeryproject.org/en/latest/userguide/routing.html / Since Celery doesn't support per-task priorities, use multiple queues and create empty queues as needed with http://docs.celeryproject.org/en/latest/configuration.html#celery-create-missing-queues / http://kombu.readthedocs.org/en/latest/userguide/examples.html#task-queue-example
• [ ] Smart queue eviction / reordering when overloaded

Heuristics

• [x] Down (All the children tiles)
• [x] Up (all the parent tiles)
• [x] Nearby (all the adjacent tiles at the same zoom level)