This introduces reviewboard.broker, which provides official support in

Review Board for communicating with message brokers (such as RabbitMQ)

and sending tasks or broadcasts to any workers connected to the broker.
This initial change offers base broker backend support, a local

filesystem-based broker, a broker registry, and the beginnings of broker

configuration.
A broker backend is responsible for connecting to a broker service,

allowing tasks to be sent or messages to be broadcast to all active

workers, and gathering worker status.
We're using Celery for the main broker work, since that's pretty

complete, but the architecture doesn't mandate this. BaseBrokerBackend

doesn't care about the transport, while BaseCeleryBrokerBackend

manages all the Celery state (and ties it to the instance, rather than

registering a global Celery instance).
Subclasses using Celery simply need to inherit from

BaseCeleryBrokerBackend and override get_celery_config() to return

the configuration and broker URI needed.
There's currently a single built-in filesystem-based broker. It stores

messages and worker registrations in the site's data directory for local

workers to access. This is a default that will be usable with a future

version of Review Bot and with an upcoming local-only companion worker

responsible for background tasks.
Future changes will implement worker scanning, more backends,

configuration, and the local background task companion worker.

Unit tests pass.
Tested the basic functionality in combination with other changes and

a modified Review Bot.