The following handler arguments are not used at all:
1. `build_id` in handlers/components.py:build_task_finalize
2. `build_name` in handlers/tags.py:tagged
Add route_task function to route celery tasks to different queues.
If we can figure out what the module build is a task ran for by
checking the task arguments, then we route this task to a queue
named:
"mbs-{}".format(module_build_id % num_workers)
"num_workers" has default value of 1, and can be changed in
backend_config.py. If module build id can't be figured out, task will
be routed to the default queue which is named "mbs-default".
While setting up the workers, the number of workers should match with
"num_workers" in config, and each worker will listen on two queues:
1. mbs-default
2. mbs-{number} # for example, the first worker listens on "mbs-0"
By this design, all tasks for a particular module build will be routed
to the same queue and run on the same worker serially.
