Use a custom Task Router to route tasks dynamically

[humitos]

This PR is an example using a Celery Task Router (under a Feature flag for now) to route tasks based on some project properties and queue's length. I think this could help us to get the most of the build:default queue without loosing performance on projects that don't really fit there and "auto-scale" projects to a bigger server when building time is getting closer to the limit.

The logic behind this, is that it will route the task to build:large in any of these cases:

1. the project is using conda
2. new project with less than N successful builds
3. last N successful builds have a high time average

(it ignores projects that have already set build_queue attribute)

These are just three examples that I thought could be useful, but we can discuss them and add others.

I tested it with our current Docker Compose setup and I does the routing properly.

[ericholscher]

We should standardize on one way to return the deafult queue. I prefer returning None, and letting Celery handle it, since it reduces our logic.

[ericholscher]

I'm 👍 on merging & shipping this once tests are fixed.